立一个flag(剪枝)

/**
 * 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 *res;
    bool flag;
    void dfs(TreeNode *root, int &k){
        if(!root || flag)   return ;
        dfs(root->left, k);
        k -- ;
        if(k == 0){
            flag = true;
            res = root;
            return ;
        }
        dfs(root->right, k);
    }
    TreeNode* kthNode(TreeNode* root, int k) {
        flag = false;
        dfs(root, k);
        return res;
    }
};