Flatten Binary Tree to Linked List

来源：Leetcode

原帖：https://oj.leetcode.com/problems/flatten-binary-tree-to-linked-list/

题目：
Given a binary tree, flatten it to a linked list in-place.
For example,
Given
      1
     /  \
   2    5
  /  \     \
 3   4    6
The flattened tree should look like:
 1
   \
    2
      \
       3
         \
          4
            \
             5
               \
                6
 Hints:
 If you notice carefully in the flattened tree, each node's right child points to the next node
 of a pre-order traversal.

代码：

 /**  
  * Definition for binary tree  
  * struct TreeNode {  
  *   int val;  
  *   TreeNode *left;  
  *   TreeNode *right;  
  *   TreeNode(int x) : val(x), left(NULL), right(NULL) {}  
  * };  
  */  
 class Solution {  
 public:  
     void flattenHelper(TreeNode *root, TreeNode* &end) {  
         if (!root) return;  
         TreeNode *lend = NULL;  
         TreeNode *rend = NULL;  
         flattenHelper(root->left, lend);  
         flattenHelper(root->right, rend);  
         if (root->left) {  
             lend->right = root->right;  
             root->right = root->left;  
             root->left = NULL;  
         }  
         end = rend ? rend : (lend ? lend : root);  
     }  
   
     void flatten(TreeNode *root) {  
         TreeNode *end = NULL; // last node in linked list  
         flattenHelper(root, end);  
     }  
 };