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;
        vector<TreeNode*> v;
        flatrec(v,root);
        TreeNode* head=v[0];
        TreeNode* p=head;
        for(int i=1;i<v.size();i++)
        {
            p->left=NULL;
            p->right=v[i];
            p=v[i];
        }
        root=head;
    }
    void flatrec(vector<TreeNode*>& v,TreeNode* root)
    {
        if(root==NULL) return;
        v.push_back(root);
        flatrec(v,root->left);
        flatrec(v,root->right);
    }
};