[LeetCode]Flatten Binary Tree to Linked List

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

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.

/**
 * 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) {
        if(root==NULL) return;
        if(root->left==NULL&&root->right==NULL) return;
        flatten(root->left);
        flatten(root->right);
        TreeNode *p=root->right;
        root->right=root->left;
        root->left=NULL;
        TreeNode *r=root;
        while(r->right)
        {
            r=r->right;
        }
        r->right=p;
    }
};