Flatten Binary Tree to Linked List

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
 //中序遍历的变种
class Solution {
public:
    void flatten(TreeNode* root) {
        if(!root)    return;
        
        flatten(root->left);
        
        TreeNode* tmp_right = root->right;
        
        if(root->left){
            root->right       = root->left;
            root->left        = NULL;
            while(root->right){
                root = root->right;
            }
            root->right = tmp_right;
        } 
        
        flatten(tmp_right);
    }
};