题解 | #第k轻的牛牛#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *二叉树的中序遍历即为升序顺序，适用栈进行中序遍历取第 K个值即可
     * @param root TreeNode类 
     * @param k int整型 
     * @return int整型
     */
    public int kthLighest (TreeNode root, int k) {
    
       Stack<TreeNode> stack = new Stack<>();
       TreeNode node = root;
       int res = root.val;
       while(!stack.isEmpty() || node != null){
           if(node != null){
            stack.add(node);
            node = node.left;
           }else {
            TreeNode temp = stack.pop();
            k--;
            if(k == 0){
                res = temp.val;
                break;
            }
            node = temp.right;
           }
       }

      return res;
    }
}