树_数据结构

二叉树基本数据结构

[相同树](#100.Same Tree)
[二叉树的层序遍历](#102. Binary Tree Level Order Traversal)
[数组转换为二分树](#108. Convert Sorted Array to Binary Search Tree)
[树的非递归遍历](#94Binary Tree Inorder Traversal)
[树的非递归遍历version2](#Binary Tree with loop)
[不同的二叉树](#96 Unique Binary Search TreesI)
[实现不同的二叉树](#95Unique Binary Search Trees II)
[高度平衡二叉树](#110Balanced Binary Tree)
[二叉查找树中查找](#700Search in a Binary Search Tree)
[根据前序和中序遍历生成二叉树](#105.Construct Binary Tree with preorder and inorder traversal)
[根据中序个后序遍历生成二叉树](#106.Construct Binary tree with inorder and postorder traversal)
[将二叉树变成一个链表形式二叉树](#114.Flatten Binary tree To Linked List)
[N个孩子结点用数组表示的树的前序非递归遍历](#589.N-ary Tree PreOrder Traversal)
[872.叶子结点相同的树](#872.Leaf similar trees)
[129.所有根结点的路径和](#129.Sum Root to Leaf Numbers)
[226.反转二叉树](#226.Invert Binary Tree)
线索二叉树:
n个结点的二叉链表每个结点有左右孩子两个指针域，有2n个指针域 ;n-1条分支线树。

100.Same Tree

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */

class Solution {
public:
    bool isSameTree(TreeNode* p, TreeNode* q) {
        if(p==NULL && q==NULL)
            return true;
        else if((p==NULL && q!=NULL)||(p!=NULL && q==NULL))
            return false;
        if(p->val==q->val){
            return isSameTree(p->left,q->left) && isSameTree(p->right,q->right);
        
        }
        return false;
        
    }
   
};

102. Binary Tree Level Order Traversal

// 二叉树的层序遍历是用数据结构 队列queue 来实现的
// 满足基本的先进先出的的顺序
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
// 利用上述的结构定义
// 将结点按层序结构依次打印
// 判断程序结束的语句还是有一点欠缺
void levelOrderTraverse(TreeNode *root){
    queue<TreeNode*> q;
    if(root)
        q.push(root);
    while(!q.empty()){
        TreeNode front=q.front();
        cout<<front->val<<endl;
        q.pop();
        if(front->left)
            q.push(front->left);
        if(front->right)
            q.push(front->right);
    }
}
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> ans;
        DFS(root,0,ans);
        return ans;
    }
private:
    void DFS(TreeNode* root,int depth,vector<vector<int>>& ans){
        if(!root) return;
        while (ans.size()<=depth) ans.push_back({});
        ans[depth].push_back(root->val);
        
        DFS(root->left,depth+1,ans);
        
        DFS(root->right,depth+1,ans);

    }
};

// 这个是由底向上输出的版本
// 我写了一个swap函数，利用循环将最后一元素pop后，插入到最前面的位置
// 显示的错误是void函数问题
// 这个问题还是模棱两可的，不是很清晰
class Solution {
public:
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        vector<vector<int>> result;
        DFS(root,0,result);
        reverse(result.begin(),result.end());  //添加一个reverse的内置函数
        return result;
    }
private:
    void DFS(TreeNode *root,int depth,vector<vector<int>>&result){
        if(!root)
            return;
        while(result.size()<=depth){
            result.push_back({});      
        }
        result[depth].push_back(root->val);
        DFS(root->left,depth+1,result);
        DFS(root->right,depth+1,result);
    }    
};

# 这个是自低向上的输出
# dfs recursively
def levelOrderBottom1(self, root):
    res = []
    self.dfs(root, 0, res)
    return res

def dfs(self, root, level, res):
    if root:
        if len(res) < level + 1:
            res.insert(0, [])
        # 保证插入的一定位于头位置
        res[-(level+1)].append(root.val)
        self.dfs(root.left, level+1, res)
        self.dfs(root.right, level+1, res)

108. Convert Sorted Array to Binary Search Tree

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */

// @param: input a array 
// @return treenode root

// divide and conquer  classsic
class Solution {
public:
    TreeNode* sortedArrayToBST(vector<int>& nums) {
        return Divide_Array(0,nums.size()-1,nums);
        
    }

    TreeNode* Divide_Array(int i,int j,vector<int>& nums){
        if(i<0||j>=nums.size()||i>j||i>=nums.size()||j<0){
            return NULL;
        }
        int mid=i+(j-i)/2;
        TreeNode* root=new TreeNode(nums[mid]);
        root->left=Divide_Array(i,mid-1,nums);
        root->right=Divide_Array(mid+1,j,nums);
        return root;
    }
    
};

二叉树前序非递归遍历

94Binary Tree Inorder Traversal

#include<stack>
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
// need stack data-structure
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        
        TreeNode *cur=root;
        //define a stack  ?
        stack<TreeNode *> s;
        vector<int> ans;
        while(cur){
            if(cur->left){
                s.push(cur);
                cur=cur->left;
            }
            else{
                ans.push_back(cur->val);
                cur=cur->right;
                
            }
            while(!cur && !s.empty()){
                cur=s.top();
                ans.push_back(cur->val);
                s.pop();
                cur=cur->right;
            }
        }
        return ans;
        
        
        
    }
};

中序遍历先考虑leftnode 左结点是不是存在，再将结点进行进栈。（左根右）

// define the stack data-structure
typedef struct Node{
    BTree data;
    struct Node *next;
}Node,*pNode;

typedef struct Stack{
    pNode pTop;
    pNode pBottom;
}Stack,*PSTACK;

// define the tree node data-structure
typedef struct BTNode{
    char data;
    struct BTNode *pLchild;
    struct BTNode *pRchild;
    
}BTNode,*BTree;

void inorder_traverse(BTree pTree){
    PSTACK stack=create_stack();
    BTree node_pop;
    BTree pcur=pTree;
    while(pcur || !is_empty(stack)){
        if(pcur->pLchild){
            push_stack(stack, pcur);
            pcur=pcur->pLchild;
        
        }
        else{
            printf("%c",pcur->data);
            pcur=pcur->pRchild;
        }
        while(!pcur && !is_empty(stack)){
            pcur=get_top(stack);
            printf("%c",pcur->data);
            pop_stack(stack, &node_pop);
            pcur=pcur->pRchild;
        }
    }
}

后序遍历是现将结点进栈，检查左右结点都是否存在，再考虑将右结点，左结点依次进栈。

// 基础的数据结构实现如中序遍历
void posorder_traverse(BTree pTree){
    PSTACK stack=create_stack();
    BTree node_pop;
    BTree pcur=pTree;
    BTree pPre=NULL;
    // first need push the first to the stack
    push_stack(stack, pTree);
    while(!is_empty(stack)){
        pcur=get_top(stack);
        // detection the node
        if((pcur->pLchild==NULL && pcur->pRchild)||(pPre!=NULL &&(pcur->pLchild==pPre ||pcur->pRchild==pPre))){
            printf("%c",pcur->data);
            pop_stack(stack,&node_pop);
            pPre=pcur;
        }
        else{
            // other situtation,mean have left and right child
            if(pcur->pRchild!=NULL)
                push_stack(stack, pcur->pRchild);
            if(pcur->pLchild!=NULL)
                push_stack(stack, pcur->pLchild);
        }
        
    }
}

Binary Tree with loop-version 2

/*
 * Date:nov.5.2018
 * @author:Danny 
 * Program function:implement traverse the tree with loop 
 */

#include<cstdlib>
using namespace std;
/*
 * Define the TreeNode data structure
 */

struct TreeNode
{
    int val;
    TreeNode *Lchild;
    TreeNode *Rchlid;
};

/*
 * 1.PreOrderTraverse
 */

/*
 * 遍历顺序：根-左-右
 * 现将根结点进行压栈
 * 当栈不为空的时候，栈顶元素出栈，并输出栈顶元素的值(根结点)
 * 如果右孩子结点存在，进行压栈
 * 如果左孩子结点存在，进行压栈
 */
void PreOrderTraverse(TreeNode *root){
    if(!root){
        cout<<"empty tree !"<<endl;
        return ;
    }
    stack<TreeNode *> stack_node;
    stack_node.push(root);
    while(!stack_node.empty()){
        TreeNode *temp=stack_node.top();
        stack_node.pop();
        cout<<temp->val<<endl;  // 现将根结点进行输出
        if(temp->Rchlid){
            stack_node.push(temp->Rchlid);
        }
        if(temp->Lchild){
            stack_node.push(temp->Lchild);
        }
    }

}

/*
 * 2.inOrderTraverse
 */

/*
 * 遍历顺序：左-根-右
 * 初始结点为根结点，如果结点不为空，将当前结点进行压栈
 * 将结点替换为结点的左孩子
 * 否则输出栈顶的元素
 * 当前结点替换为结点的右孩子，出栈
 */

void inOrderTraverse(TreeNode *root){
    if(!root){
        cout<<"empty tree!"<<endl;
        return ;
    }
    stack<TreeNode *>stack_node;
    while(root|| !stack_node.empty()){
        if(root){
            stack_node.push(root);
            root=root->Lchild;
        }
        else{
            TreeNode *cur=stack_node.top();
            stack_node.pop();
            cout<<cur->val<<endl;
            root=cur->Rchlid;
        }
    }
}

/*
 * 3.PostOrderTraverse
 */

/*
 * 遍历顺序：左-右-根
 * 由两个栈来实现
 * 先把根结点压入第一个栈
 */

void PostOrderTraverse(TreeNode *root){
    if(!root){
        cout<<"empty tree !"<<endl;
        return ;
    }
    stack<TreeNode *>stack1,stack2;
    stack1.push(root);
    while(!stack1.empty()){
        TreeNode *cur=stack1.top();
        stack1.pop();
        stack2.push(cur);
        if(cur->Lchild)
            stack1.push(cur-<Lchild);
        if(cur->Rchlid)
            stack1.push(cur->Rchlid);
    }
    while(!stack2.empty()){
        TreeNode *top=stack2.top();
        stack2.pop();
        cout<<top->val<<endl;
    }

}

96 Unique Binary Search Trees

前序遍历是将根结点进展，在考虑左结点是否存在。（根左右）

class Solution {
public:
    int numTrees(int n) {
        vector<int> sub_tree(n+1,0);
        sub_tree[0]=1;
        for(int i=1;i<=n;i++){
            for(int j=0;j<i;j++){
                sub_tree[i]+=sub_tree[j]*sub_tree[i-j-1];
            }
        }
        return sub_tree[n];
        
        
        // formula derivation error
        /*
        if(n==0)
            return 0;
        if(n==1)
            return 1;
        while(n>=2){
            return 2*(numTrees(n-1)+numTrees(n-2));
        }
        */
        
    }
};

95Unique Binary Search Trees II

-这个代码是没有看懂的，需要后续继续补充学习

// create :2018.10.12
// @author:dengshuo

class Solution {
private:
    vector<TreeNode*> helper(int start, int end){
        vector<TreeNode*> res;
        if(start > end) {
            res.push_back(NULL);
            return res;
        }
        // use two pointer 
        for(int i = start; i <= end; i++){
            vector<TreeNode*> lefts = helper(start, i - 1);
            vector<TreeNode*> rights = helper(i + 1, end);
            for(int j = 0; j < (int)lefts.size(); j++){
                for(int k = 0; k < (int)rights.size(); k++){
                    TreeNode* root = new TreeNode(i);
                    root->left = lefts[j];
                    root->right = rights[k];
                    res.push_back(root);
                }
            }
        }
        return res;
    }
public:
    vector<TreeNode*> generateTrees(int n) {
        if(n == 0) return vector<TreeNode*>(0);
        return helper(1,n);
    }
};

// Author: Huahua
// Running time: 16 ms
class Solution {
public:
  vector<TreeNode*> generateTrees(int n) {
    if (n == 0) return {};
    const auto& ans = generateTrees(1, n);
    cout << ans.size() << endl;     // 计算结点数
    return ans;
  }
private:
  vector<TreeNode*> generateTrees(int l, int r) {
    if (l > r) return { nullptr };
    vector<TreeNode*> ans;
    for (int i = l; i <= r; ++i)
      for (TreeNode* left : generateTrees(l, i - 1))
        for (TreeNode* right : generateTrees(i + 1, r)) {
          ans.push_back(new TreeNode(i));
          ans.back()->left = left;
          ans.back()->right = right;
        }
    return ans;
  }
};

110Balanced Binary Tree

实现高度平衡二叉树
平衡二叉树：
结点为空或者左右两个子树高度差的绝对值不超过1.
还是使用递归来实现，要注意这个递归实现的顺序流程

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isBalanced(TreeNode* root) {
        bool flags=true;              //定义一个贯穿运行始终的旗帜变量。如果不定义 对于私有函数无法传递变量
        lrdepth(root,flags);
        return flags;
        }
private:
    int lrdepth(TreeNode* root,bool &flags){
        if(!root)
            return 0;
        int l_num=lrdepth(root->left,flags);
        int r_num=lrdepth(root->right,flags);
        if(abs(l_num-r_num)>1){
            flags=false;
        }
        return 1+max(l_num,r_num);
    }
};

700.Search in a Binary Search Tree

class Solution {
public:
    TreeNode* searchBST(TreeNode* root, int val) { 
        if(root==NULL || root->val==val)
            return root;                                             //当树为空的时候，返回 [] 不知道为什么这个还是可以运行。
        if(val<root->val)
            return searchBST(root->left,val);
        else{
            return searchBST(root->right,val);
        }
    }
};

105.Construct Binary Tree with preorder and inorder traversal

// 这就是我的思路
// 根据preorder确定当前根结点的位置
// 在根据 根结点在inorder的位置，划分左右子树
// 递归的进行，知道左右子树都为空，或者前序和后序遍历为空截止递归
class Solution {
public:
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        if(perorder.size()==0 || inorder.size()==0)
            return NULL;
        int index=                           //我找不出根结点在中序遍历的位置，无法进行划分
        TreeNode *root=preorder[0];
        root->left=bulidTree();
        root->right=bulidTree();        
    }
};
//添加辅助函数
TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
    return builTreeHelper(preorder,0,preorder.size(),inorder,0,inorder.size());
}

TreeNode* builTreeHelper(vector<int>& preorder, int sp, int ep, vector<int>& inorder, int si, int ei) {
    if (sp == ep) return nullptr;
    TreeNode* root = new TreeNode(preorder[sp]);
    int dis = find(inorder.begin()+si,inorder.begin()+ei,preorder[sp]) - inorder.begin() - si;
    root->left = builTreeHelper(preorder,sp+1,sp+1+dis,inorder,si,si+dis);
    root->right = builTreeHelper(preorder,sp+1+dis,ep,inorder,si+dis+1,ei);
    return root;
}
// 未通过的代码
// 同样也是使用辅助函数来实现
class Solution {
public:
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        if(preorder.size()==0 || inorder.size()==0)
            return NULL;
        return recursionBulid(preorder,0,preorder.size()-1,inorder,0,inorder.size()-1);
    }
private:
    TreeNode *recursionBulid(vector<int>& preorder,int startpreorder,int endpreorder,vector<int>& inorder,int startinorder,int endinorder){
        int rootvalue=preorder[startpreorder];
        TreeNode *root= new TreeNode(0);
        root->val=rootvalue;
        root->left=root->right=NULL;
        if(startpreorder==endpreorder){
            if(startinorder==endinorder)
                return root;
        }
        int root_inorder=startpreorder;
        while(root_inorder<=endinorder && inorder[root_inorder]!=rootvalue){
            root_inorder++;
        }
        int root_left_length=root_inorder-startpreorder;
        int left_preorder_end=startpreorder+root_left_length;
        if(root_left_length>0){
            root->left=recursionBulid(preorder,startpreorder+1,left_preorder_end,inorder,startinorder,root_inorder-1);
        }
        if(root_left_length<endpreorder-startpreorder){
            root->right=recursionBulid(preorder,left_preorder_end+1,endpreorder,inorder,root_inorder+1,endinorder);
        }
        return root;       
    }
};

106.Construct Binary tree with inorder and postorder traversal

// 按照自己的思路来写的
// Runtime error
// 错可能就错在递归函数的边界值。能够取到，是从什么开始
// 例如 取左值，不取右值。可以对比下面一个代码的区间问题
class Solution {
public:
    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
        // can construct a binary tree
        if(inorder.size()==0 || postorder.size()==0){
            return nullptr;
        }
        if(inorder.size()==1 && postorder.size()==1){
            TreeNode * root=new TreeNode(postorder[0]);
            return root;
        }
        return recursionBulid(inorder,0,inorder.size()-1,postorder,0,postorder.size()-1);     
    }
private:
    TreeNode *recursionBulid(vector<int>& inorder,int start_in,int end_in,vector<int> &postorder,int start_post,int end_post){
        TreeNode *root=new TreeNode(postorder[end_post]);
        int root_inorder=start_in;
        int root_value=postorder[end_post];
        while(root_inorder<end_in && inorder[root_inorder]!=root_value){
            root_inorder++;
        }
        int left_length=root_inorder-start_in;
        root->left=recursionBulid(inorder,start_in,root_inorder-1,postorder,start_post,left_length-start_post);
        root->right=recursionBulid(inorder,root_inorder+1,end_in,postorder,left_length+1,end_post-1);
        return root;
    }
};
// 利用vecotr 可以迭代的性质
// 注意便捷的情况
public:
    typedef vector<int>::iterator Iter;
    TreeNode *buildTree(vector<int> &inorder, vector<int> &postorder) {
        // write your code here
        if (inorder.size() == 0)
            return NULL;
        return buildTreeRecur(inorder.begin(), inorder.end(), postorder.begin(), 
        postorder.end());
    }
    TreeNode *buildTreeRecur(Iter istart, Iter iend, Iter pstart, Iter pend)
    {
        if(istart == iend)return NULL;
        int rootval = *(pend-1);
        Iter iterroot = find(istart, iend, rootval);
        TreeNode *res = new TreeNode(rootval);
        res->left = buildTreeRecur(istart, iterroot, pstart, pstart+(iterroot-istart));
        res->right = buildTreeRecur(iterroot+1, iend, pstart+(iterroot-istart), pend-1);
        return res;
    }
};

114.Flatten Binary tree To Linked List

// 这是自己写的
// 思路：
// 1.利用前序遍历preorder，遍历所有结点
// 2.构造二叉树，一直添加右子结点

// 这个代码运行时错误的
class Solution {
public:
    void flatten(TreeNode* root) {
        queue<TreeNode*> q=preOrder(root);
        q.pop();
        while(!q.empty()){
            root->left=NULL;
            root->right=q.front();
            q.pop();
            root=q.front();
        }          
    }
private:
    queue<TreeNode*>preOrder(TreeNode *root){
        stack<TreeNode *> s;
        queue<TreeNode *> q;
        s.push(root);
        while(!s.empty()){
            TreeNode *temp=s.top();
            s.pop();
            q.push(temp);
            if(temp->right){
                s.push(temp->right);
            }
            if(temp->left){
                s.push(temp->left);
            }
        }
        return q;
    }
};

// 参考别人的代码
// 这个写的真的是很漂亮的代码
// 直接利用二叉树的性质来进行结点相互交换完成排序的实现
// 真的牛皮！！！！！
class Solution {
public:
    void flatten(TreeNode *root) {
        TreeNode*now = root;
        while (now)
        {
            if(now->left)
            {
                //Find current node's prenode that links to current node's right subtree
                TreeNode* pre = now->left;
                while(pre->right)
                {
                    pre = pre->right;
                }
                pre->right = now->right;
                //Use current node's left subtree to replace its right subtree(original right 
                //subtree is already linked by current node's prenode
                now->right = now->left;
                now->left = NULL;
            }
            now = now->right;
        }
    }
};

589.N-ary Tree PreOrder Traversal

/*
思路清晰：
1.利用栈来实现树的前序遍历
2.对于孩子结点，进行pop_back()进栈
代码正确：
就是root不存在的返回值存在一点小问题
 直接返回 定义值（未初始化的值）
*/
// 注意 结点的定义发生了变化
/*
// Definition for a Node.
class Node {
public:
    int val;
    vector<Node*> children;

    Node() {}

    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/
class Solution {
public:
    vector<int> preorder(Node* root) {
        vector<int> result;
        stack<Node*> s;       // 利用栈来实现树的前序遍历
        s.push(root);
        if(!root)
            return result;
        while(!s.empty())
        {
            Node *temp=s.top();
            s.pop();
            result.push_back(temp->val);  
            while(!temp->children.empty())。// 返回孩子结点中的所有孩子
            {
                s.push(temp->children.back()); // 压缩到栈中
                temp->children.pop_back();
            }
        }   
        return result;       
    }
};

872.Leaf similar trees

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
// 也可以解决，但是容易出现问题
class Solution {
public:
    bool leafSimilar(TreeNode* root1, TreeNode* root2) {
        return Leaf(root1)==Leaf(root2)
        
    }
    vector<int> preorder(TreeNode *root){
        vector<int> leafnode;
        if(!root)
            return leafnode;
        if(!root->left && !root->right){
            leafnode.push_back(root->val);
        }
        preorder(root->left);
        preorder(root->right);
        
        return leafnode;
        
    }
};

// 
// 最大的不同就在于，将参数放在函数中
// 函数是一个无返回的函数
class Solution {
public:
    bool leafSimilar(TreeNode* root1, TreeNode* root2) {
        vector<int> leafnode1,leafnode2;
        preorder(root1,leafnode1);
        preorder(root2,leafnode2);
        return leafnode1==leafnode2;
        
    }
    void preorder(TreeNode *root,vector<int> &leafnode){
        if(!root)
            return ;
        if(!root->left && !root->right){
            leafnode.push_back(root->val);
        }
        preorder(root->left,leafnode);
        preorder(root->right,leafnode);       
    }
};

129.Sum Root to Leaf Numbers

// 理解错误，没有分开左右子树
// 这个是计算所有结点(前序)的路径和
class Solution {
public:
    int sumNumbers(TreeNode* root) {
        int sum=0;
        preorder(root,sum);
        return sum;             
    }
    void preorder(TreeNode *root,int &sum){
        if(!root)
            return ;
        sum=sum*10+root->val;
        preorder(root->left,sum);
        preorder(root->right,sum);
    }
};
// 正确版本
// 就是思考的方向不对
// 1.要分左右子树
// 2.现考虑左右子树都不存在的情况
class Solution {
public:
    int sumNumbers(TreeNode* root) {
        if(!root)
            return 0;
        return sumAll(root,0);
    }
    int sumAll(TreeNode *root,int x){
        if(root->left==NULL && root->right==NULL)
            return x*10+root->val;
        int lrSum=0;           // 这个用来计算左右子树的总和
        if(root->left!=NULL)
            lrSum+=sumAll(root->left,x*10+root->val);
        if(root->right!=NULL)
            lrSum+=sumAll(root->right,x*10+root->val);
        return lrSum;
    }
};

226.Invert Binary Tree

// 这道题的思想其实很简单
// 就是最基本的DFS，深度优先遍历
// 先解决一个一个分支，这个思想我还是有点缺乏
// 自己写的
class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if(!root)
            return root;
        TreeNode *var=root;
        Helper(var);
        return root;
    }
    void Helper(TreeNode *var){
        // 再写个结束条件
        TreeNode *temp=var->left;
        var->left=var->right;
        var->right=temp;  
        Helper(var->left);
        Helper(var->right); 
    }      
};
// 这个是正确利用深度优先遍历的代码
class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if(!root)
            return root;
        if(!root->left && !root->right)
            return root;
        TreeNode *temp;
        temp=invertTree(root->left);
        root->left=invertTree(root->right);
        root->right=temp;
        return root;
    }      
};

不要用狭隘的眼光看待不了解的事物，自己没有涉及到的领域不要急于否定．每天学习一点，努力过好平凡的生活．

树_数据结构

二叉树基本数据结构

目录

100.Same Tree

102. Binary Tree Level Order Traversal

108. Convert Sorted Array to Binary Search Tree

94Binary Tree Inorder Traversal

中序遍历先考虑leftnode 左结点是不是存在，再将结点进行进栈。（左根右）

后序遍历是现将结点进栈，检查左右结点都是否存在，再考虑将右结点，左结点依次进栈。

Binary Tree with loop-version 2

96 Unique Binary Search Trees

前序遍历是将根结点进展，在考虑左结点是否存在。（根左右）

95Unique Binary Search Trees II

110Balanced Binary Tree

700.Search in a Binary Search Tree

105.Construct Binary Tree with preorder and inorder traversal

106.Construct Binary tree with inorder and postorder traversal

114.Flatten Binary tree To Linked List

589.N-ary Tree PreOrder Traversal

872.Leaf similar trees

129.Sum Root to Leaf Numbers

226.Invert Binary Tree