[leetCode]Flatten Binary Tree to Linked List

递归的思想，左子树整理成一条链，右子树整理成一条链，然后左子树的尾节点链上右子树的头节点就好了

 1 #include <iostream>
 2 using namespace std;
 3 struct TreeNode {
 4     int val;
 5     TreeNode  *left;
 6     TreeNode  *right;
 7     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8 };
 9 class Solution {
10 public:
11     void flatten(TreeNode *root) {
12         if(root == NULL) return;
13         flattenTree(root);
14     }
15     TreeNode *flattenTree(TreeNode *root){
16         if(root == NULL) return NULL;
17         if(root->left == NULL && root->right == NULL) return root;
18         TreeNode *lefthead,*righthead;
19         righthead = lefthead = NULL;
20         if(root->left != NULL) lefthead = flattenTree(root->left);
21         if(root->right != NULL) righthead = flattenTree(root->right);
22         if(lefthead == NULL) root->right = righthead;
23         if(righthead == NULL) root->right = lefthead;
24         if(lefthead != NULL && righthead != NULL){
25             root->right = lefthead;
26             TreeNode *leftTail = getTail(lefthead);
27             leftTail->right = righthead;
28         }
29         root->left = NULL;
30         return root;
31     }
32     TreeNode *getTail(TreeNode *root){
33         if(root == NULL) return NULL;
34         while(root->right != NULL) root = root->right;
35         return root;
36     }
37 };

艰难的成长