【leetcode刷题】T130-二叉搜索树中第K小的元素

示例 1:
输入: root = [3,1,4,null,2], k = 1
   3
  / \
 1   4
  \
   2
输出: 1
示例 2:
输入: root = [5,3,6,2,4,null,null,1], k = 3
       5
      / \
     3   6
    / \
   2   4
  /
 1
输出: 3

/**
 * 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:
    int kthSmallest(TreeNode* root, int k) {
        int res;
        find(root, k, res);
        return res;
    }

    void find(TreeNode* root, int& k, int& res){
        if(root->left)
            find(root->left, k, res);
        if(--k == 0){
            res = root->val;
            return;
        }
        if(root->right)
            find(root->right, k, res);
    }
};