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
     *
     *
     * @param root TreeNode类
     * @param k int整型
     * @return int整型
     */
    public int kthLighest (TreeNode root, int k) {
        // write code here
        ArrayList<Integer> arrayList = new ArrayList<>();
        inorder(root, arrayList);
        return arrayList.get(k - 1);
    }

    public void inorder (TreeNode root, ArrayList<Integer> arrayList) {
        if (root != null) {
            inorder(root.left, arrayList);
            arrayList.add(root.val);
            inorder(root.right, arrayList);
        }
    }
}