leetCode159|二叉搜索树的第k大节点

示例 1:

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

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

限制：

1 ≤ k ≤ 二叉搜索树元素个数

  

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;

public class KthLargestTest3 {
    public static void main(String[] args) {
        TreeNode t1=new TreeNode(3);
        TreeNode t2=new TreeNode(1);
        TreeNode t3=new TreeNode(4);
        TreeNode t4=new TreeNode(2);
        t1.left=t2;
        t1.right=t3;
        t2.right=t4;
        int k=1;
        int kthLargest = kthLargest(t1, k);
        System.out.println("kthLargest = " + kthLargest);

    }

    public static  int kthLargest(TreeNode root, int k) {
        if (root == null) {
            return 0;
        }
        List<Integer> list = new ArrayList<>();
        dfs(root, list);
        list.sort(Comparator.reverseOrder());
       
        return list.get(k - 1);
    }

    private static  void  dfs(TreeNode root, List<Integer> list) {
        if (root == null) {
            return;
        }
        if (root.left != null) {
            dfs(root.left, list);
        }
        list.add(root.val);
        if (root.right != null) {
            dfs(root.right, list);
        }
    }
}