代码随想录算法刷题训练营day20：LeetCode(654)最大二叉树、LeetCode(617)合并二叉树、LeetCode(700)二叉搜索树中的搜索、LeetCode(700)二叉搜索树中的搜索、LeetCode(98)验证二叉搜索

LeetCode(654)最大二叉树
题目

代码

import java.util.Arrays;/*** 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 constructMaximumBinaryTree(int[] nums) {//递归----判断终止条件if(nums.length==0){return null;}int data[]=getMaxIndexAndValue(nums);int dataIndex=data[0];int dataValue=data[1];TreeNode root=new TreeNode(dataValue);//拆分数组int[] leftNums=Arrays.copyOfRange(nums, 0, dataIndex);int[] rightNums=Arrays.copyOfRange(nums, dataIndex+1, nums.length);root.left=constructMaximumBinaryTree(leftNums);//左边构建左子树root.right=constructMaximumBinaryTree(rightNums);//右边构建右子树return root;}//定义一个函数用于获取数组中的最大值和对应的数组下标public int[] getMaxIndexAndValue(int[] nums){int[] result=new int[2];int index=0;int sum=0;for (int i = 0; i < nums.length; i++) {if(nums[i]>sum){sum=nums[i];index=i;}    }result[0]=index;result[1]=sum;//记住最大值的下标和对应的值return result;}
}

LeetCode(617)合并二叉树
题目

代码

/*** 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 mergeTrees(TreeNode root1, TreeNode root2) {//先处理边角条件if(root1==null&&root2==null){return null;}if(root1==null&&root2!=null){return root2;}if(root1!=null&&root2==null){return root1;}TreeNode root=getMergeTrees(root1,root2);return root;}//单独构建一个函数用于处理public TreeNode getMergeTrees(TreeNode root1,TreeNode root2){//终止条件if(root1==null&&root2==null){return null;}/* if(root1==null){return root2;//树1为空即返回树2}if(root2==null){return root1;} */TreeNode root=new TreeNode();if(root1!=null&&root2==null){/* root.val=root1.val;return root; *///还会往下面走，所以空指针异常/* root.val=root1.val; */return root1;//树二已经没有了，需要把树1所有子树全部返回//}if(root1==null&&root2!=null){/* root.val=root2.val;return root; */return root2;}//先序遍历----同时遍历到同一位置if(root1!=null&&root2!=null){int data=root1.val+root2.val;root.val=data;}//左子树遍历root.left=getMergeTrees(root1.left, root2.left);//右子树遍历root.right=getMergeTrees(root1.right, root2.right);return root;}
}

LeetCode(700)二叉搜索树中的搜索
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代码

/*** 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;*     }* }*/
//二叉平衡树
class Solution {public TreeNode searchBST(TreeNode root, int val) {//递归----判断终止条件if(root==null){return null;}//前序遍历if(root.val==val){return root;}TreeNode result=new TreeNode();//左边//左子树//二叉搜索树//左边小于中间，右边大于中间if(val<root.val){//往左边走result=searchBST(root.left, val);}if(val>root.val){//右子树result=searchBST(root.right, val);}return result;}
}

LeetCode(98)验证二叉搜索
题目

代码

import java.util.ArrayList;
import java.util.List;/*** 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 boolean isValidBST(TreeNode root) {//定义一个集合，遍历树存储到集合里面//按中序遍历存储数据，若是二叉搜索树，则是一个升序数组List<Integer> dateTree=new ArrayList<>();getDateTree(root,dateTree);for (int i = 0; i < dateTree.size()-1; i++) {if(dateTree.get(i)>=dateTree.get(i+1)){return false;}   }return true;}public void getDateTree(TreeNode root,List<Integer> dateTree){if(root==null){return;}getDateTree(root.left, dateTree);//左子树dateTree.add(root.val);getDateTree(root.right, dateTree);//右子树}}