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)