leetcode 114. 二叉树展开为链表(dfs)

/**
 * 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:
    TreeNode* h;
    void dfs(TreeNode* root){
        if(root!=NULL){
            if(root->right!=NULL)dfs(root->right);
            if(root->left!=NULL)dfs(root->left);
            root->right=h;
            root->left=NULL;
            h=root;
        }
    }
    void flatten(TreeNode* root) {
        h=NULL;
        dfs(root);
    }
};