leetcode 94: Validate Binary Search Tree

Validate Binary Search Tree  Aug 31 '126322 / 18211

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than the node's key.
• The right subtree of a node contains only nodes with keys greater than the node's key.
• Both the left and right subtrees must also be binary search trees.

Write a function isBST(BinaryTree *node) to verify if a given binary tree is a Binary Search Tree (BST) or not

1. every node must be greater than left node and less than right node.

2. also must be less than parent if left child or greater than parent if right child.

java 1. (NULL null) 2. (-> .) 3. ( !treenode  treenode==null) 4. (INT_MIN Integer.MIN_VALUE) 

 

 

class Solution {public:    bool isBinarySearchTree(TreeNode * root ) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        return isBST( root, INT_MIN, INT_MAX);    }        bool isBST( TreeNode* root, int Min, int Max) {        if(!root) return true;                if( root->val > Min && root->val < Max) {            return isBST( root->left, Min, root->val) && isBST( root->right, root->val, Max);        } else {            return false;        }    }};

 

java version

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public boolean isValidBST(TreeNode root) {        // Start typing your Java solution below        // DO NOT write main() function        return isValidRec(root, Integer.MIN_VALUE, Integer.MAX_VALUE);     }        public boolean isValidRec(TreeNode root, int min, int max) {        if( root==null ) return true;                int val = root.val;        if( val <= min || val >= max) return false;                return isValidRec( root.left, min, val) && isValidRec(root.right, val, max);                    }}

 

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    bool isValidBST(TreeNode *root) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        if(root==NULL) return true;        return validRec(root, INT_MIN, INT_MAX);    }private:    bool validRec(TreeNode *root, int min, int max) {        if( root->val <= min || root->val >= max) return false;        bool x=true, y=true;        if(root->left) x=validRec(root->left, min, root->val);        if(root->right) y=validRec(root->right, root->val, max);        return x && y;    }    };