力扣labuladong——一刷day69

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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);}
}