代码随想录day20

一、最大二叉树（LeetCode654）

/**
 * 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) {
        return travesal(nums,0,nums.length);
    }
    TreeNode travesal(int[] nums,int leftindex,int rightindex){
        if(rightindex - leftindex < 1){//无元素
            return null;
        }
        if(rightindex - leftindex == 1){//一个元素
            return new TreeNode(nums[leftindex]);
        }
        int index = leftindex;//最大元素所在位置
        int maxvalue = nums[index];//最大元素值
        for(int i = leftindex + 1;i<rightindex;i++){
            if(maxvalue < nums[i]){
                maxvalue = nums[i];
                index = i;
            }
        }
        TreeNode node = new TreeNode(maxvalue);
        // 根据index划分左右子树
        node.left = travesal(nums,leftindex,index);
        node.right = travesal(nums,index+1,rightindex);
        return node;
    }
}

二、合并二叉树（LeetCode617）

/**
 * 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) {
        return travesal(nums,0,nums.length);
    }
    TreeNode travesal(int[] nums,int leftindex,int rightindex){
        if(rightindex - leftindex < 1){//无元素
            return null;
        }
        if(rightindex - leftindex == 1){//一个元素
            return new TreeNode(nums[leftindex]);
        }
        int index = leftindex;//最大元素所在位置
        int maxvalue = nums[index];//最大元素值
        for(int i = leftindex + 1;i<rightindex;i++){
            if(maxvalue < nums[i]){
                maxvalue = nums[i];
                index = i;
            }
        }
        TreeNode node = new TreeNode(maxvalue);
        // 根据index划分左右子树
        node.left = travesal(nums,leftindex,index);
        node.right = travesal(nums,index+1,rightindex);
        return node;
    }
}

三、二叉搜索树中的搜索 （LeetCode700）

递归

/**
 * 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 searchBST(TreeNode root, int val) {
        if(root == null || root.val == val) return root;
        TreeNode node = null;
        if(val<root.val){
            node = searchBST(root.left,val);
        }
        if(val>root.val){
            node = searchBST(root.right,val);
        }
        return node;
    }
}

迭代

/**
 * 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 searchBST(TreeNode root, int val) {
        while (root != null)
            if (val < root.val) root = root.left;
            else if (val > root.val) root = root.right;
            else return root;
        return null;
    }
}

四、验证二叉搜索树（LeetCode98）

/**
 * 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 {
    // 递归
    long max = Long.MIN_VALUE;
    public boolean isValidBST(TreeNode root) {
        if (root == null) {
            return true;
        }
        // 左
        boolean left = isValidBST(root.left);
        // 中
        if (max < root.val) {
            max = root.val;
        }else{
            return false;
        }
        // 右
        boolean right = isValidBST(root.right);
        return left && right;
    }
}