LeetCode123|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

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

public class KthSmallestTest3 {
    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 kthSmallest = kthSmallest(t1, k);
        System.out.println("kthSmallest = " + kthSmallest);

    }

    private static List<Integer> list = new ArrayList<>();

    public static int kthSmallest(TreeNode root, int k) {
        if (root == null) {
            return 0;
        }
        dfs(root);
        return list.get(k - 1);
    }

    private static void dfs(TreeNode root) {
        if (root == null) {
            return;
        }
        if (root.left != null) {
            dfs(root.left);

        }
        list.add(root.val);
        if (root.right != null) {
            dfs(root.right);
        }
    }
}