检查是否是BST 牛客网 程序员面试金典 C++ java Python

检查是否是BST 牛客网 程序员面试金典  C++ java Python

• 题目描述
• 请实现一个函数，检查一棵二叉树是否为二叉查找树。
• 给定树的根结点指针TreeNode* root，请返回一个bool，代表该树是否为二叉查找树。

C++

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};*/

class Checker {
public:
    //run:4ms memory:472k
    bool checkBST(TreeNode* root) {
        if (NULL == root) return true;
        if (NULL != root->left){
            if (root->left->val > root->val) return false;
            if (root->left->right && (root->left->right->val > root->val)) return false;
        }
        if (NULL != root->right){
            if (root->right->val < root->val) return false;
            if (root->right->left && (root->right->left->val < root->val)) return false;
        }
        return checkBST(root->left) && checkBST(root->right);
    }
};

java

import java.util.*;

/*
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;
    public TreeNode(int val) {
        this.val = val;
    }
}*/
public class Checker {
    // run: 42ms memory:10128k
    public boolean checkBST(TreeNode root){
        if (null == root) return true;
        if (root.left != null){
            if (root.left.val > root.val) return false;
            if (root.left.right !=null && root.left.right.val > root.val) return false;
        }
        if (root.right != null){
            if(root.right.val < root.val) return false;
            if(root.right.left != null && root.right.left.val < root.val) return false;
        }
        return checkBST(root.left) && checkBST(root.right);
    }
}

Python

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Checker:
    #run:48ms memory:5852k
    def checkBST(self, root):
        if None == root: return True
        if None != root.left:
            if root.left.val > root.val: return False
            if None != root.left.right and root.left.right.val > root.val: return False
        if None != root.right:
            if root.right.val <root.val: return False
            if None != root.right.left and root.right.left.val < root.val: return False
        return self.checkBST(root.left) and self.checkBST(root.right)