day58（1.9）——leetcode面试经典150

230. 二叉搜索树中第 K 小的元素

230. 二叉搜索树中第k小的元素

题目：

题解：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    private int x = -1;
    private int t = 0;
    public void dfs(TreeNode root, int k) {
        if(root == null || x != -1) {
            return;
        }
        dfs(root.left, k);
        t++;
        if(t==k) {
            x = root.val;
        }
        dfs(root.right, k);
    }


    public int kthSmallest(TreeNode root, int k) {
        dfs(root, k);
        return x;
    }
}