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
代码：

    void flatten(TreeNode *root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(root == NULL)
            return; 
        flatten(root->left);
        flatten(root->right);
        if(root->left){
            TreeNode *tmp = root->left;
            while(tmp->right){
                tmp = tmp->right;
            }
            tmp->right = root->right;
            root->right = root->left;
        }
        root->left = NULL;
    }