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
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;
 *     }
 * }
 */
//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
    }
    
}