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.

Analyse: The sequence of the linked list is the preorder traversal. So we can first do the preorder traversal then create a new tree.

Runtime: 12ms.

/**
 * Definition for a binary tree node.
 * 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) return;
        vector<TreeNode* > vec;
        preorder(root, vec);
        
        TreeNode* current = root;
        int index = 1;
        while(index < vec.size()){
            TreeNode* temp(vec[index]);
            current->right = temp;
            current->left = NULL;
            current = temp;
            index++;
        }
        return;
    }
    void preorder(TreeNode* root, vector<TreeNode* >& vec){
        if(!root) return;
        
        vec.push_back(root);
        preorder(root->left, vec);
        preorder(root->right, vec);
    }
};