力扣labuladong——一刷day37

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前言

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介绍了 BST 的基本特性，还利用二叉搜索树「中序遍历有序」的特性来解决了几道题目，本文来实现 BST 的基础操作：判断 BST 的合法性、增、删、查。其中「删」和「判断合法性」略微复杂

一、力扣98. 验证二叉搜索树

/**
 * 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 {
    TreeNode pre = null;
    public boolean isValidBST(TreeNode root) {
        return fun(root);
    }
    public boolean fun(TreeNode root){
        if(root == null){
            return true;
        }
        boolean l = fun(root.left);
        if(pre != null && root.val <= pre.val){
            return false;
        }
        pre = root;
        boolean r = fun(root.right);
        return l && r;
    }
}

二、力扣700. 二叉搜索树中的搜索

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

三、力扣701. 二叉搜索树中的插入操作

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

四、力扣450. 删除二叉搜索树中的节点

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