[LeetCode] Flatten Binary Tree to Linked List

Given a binary tree, flatten it to a linked list in-place.

For example,
Given

The flattened tree should look like:

click to show hints.

Hints:

If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.

思路一:递归的思想。使用后序递归的方法。先将左右子树转换为链表，再将左右子树连接

　　　时间复杂度O(n),空间复杂度O(logN)

 1 /**
 2  * Definition for binary tree
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     void flatten(TreeNode *root) {
13         if (root == nullptr) return;
14         
15         flatten(root->left);
16         flatten(root->right);
17         
18         //三方合并，将左子树形成的链表插入到root和root->right之间
19         TreeNode *p = root->right;
20         while (p->right) p = p->right;
21         p->right = root->right;
22         root->right = root->left;
23         root->left = nullptr;
24     }
25 };

思路二：迭代的方法。依据题目与前序遍历之间的关系。使用前序遍历的方法。

 1 void flatten(TreeNode *root) {
 2         if(root == NULL) return;
 3     while(root){
 4         if(root->left){
 5             TreeNode *pre = root->left;
 6             while(pre->right)
 7                 pre = pre->right;
 8             pre->right = root->right;
 9             root->right = root->left;
10             root->left = NULL;
11         }
12         root = root->right;
13     }
14     }