力扣labuladong——一刷day69

提示：文章写完后，目录可以自动生成，如何生成可参考右边的帮助文档

文章目录

• 前言
• 一、力扣669. 修剪二叉搜索树
• 二、力扣671. 二叉树中第二小的节点

前言

二叉树的递归分为「遍历」和「分解问题」两种思维模式，这道题需要用到「遍历」的思维模式。

一、力扣669. 修剪二叉搜索树

/** * 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 {public TreeNode trimBST(TreeNode root, int low, int high) {if(root == null){return null;}if(root.val < low){return trimBST(root.right,low,high);}if(root.val > high){return trimBST(root.left, low, high);}root.left = trimBST(root.left, low, high);root.right = trimBST(root.right, low, high);return root;}}

二、力扣671. 二叉树中第二小的节点

/** * 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 {public int findSecondMinimumValue(TreeNode root) {if(root.left == null && root.right == null){return -1;}int left = root.left.val, right = root.right.val;if(root.val == root.left.val){left = findSecondMinimumValue(root.left);}if(root.val == root.right.val){right = findSecondMinimumValue(root.right);}if(left == -1){return right;}if(right == -1){return left;}return Math.min(left,right);}}