LeetCode 114. 二叉树展开为链表（Flatten Binary Tree to Linked List）

 1 /**
 2  * Definition for a binary tree node.
 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) flat(root);
14     }
15     TreeNode* flat(TreeNode* root){
16         if(root->left == NULL && root->right == NULL) return root;
17         TreeNode* right = root->right;
18         TreeNode* last = NULL;
19         if(root->left){
20             root->right = root->left;
21             root->left = NULL;
22             last = flat(root->right);
23             last->right = right;
24         }
25         if(right) return flat(right);
26         return last;
27     }
28 };