Leetcode -- Flatten Binary Tree to Linked List

 
/**  
  * 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 flatten(TreeNode *root) {  
     flattenHelper(root);  
   }  
   TreeNode * flattenHelper(TreeNode * root) {  
     if(root == NULL) {  
       return root;  
     }  
     TreeNode * leftFlatten = flattenHelper(root->left);  
     TreeNode * rightFlatten = flattenHelper(root->right);  
     if(leftFlatten != NULL) {  
        root->right = leftFlatten;  
       TreeNode * p = root;  
       while(p->right != NULL) {  
         p = p->right;  
       }  
       p->right = rightFlatten;  
     }   
     root->left = NULL;  //DON'T FORGET TO CLEAR LEFT TREE!!!!!
     return root;  
   }  
 };