98. Validate Binary Search Tree 🌟🌟

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

A valid 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.

O(N) Time and O(N) space

• Inorder traversal of the binary tree gives sorted array.
• we can traverse the tree inorder and check if the array is sorted or not.

O(N) Time solution

• we will check if value is less than maximum and greater than minimum
• Refer This , if you are not able to understand.

Code

class Solution{
private:
    bool valid(TreeNode *node, long long left, long long right){
        if (!node) return true;
        if (!(node->val < right && node->val > left)) return false;

        return (valid(node->left, left, node->val) &&
                (valid(node->right, node->val, right)));
    }

public:
    bool isValidBST(TreeNode *root){
        return valid(root, LONG_MIN, LONG_MAX);
    }
};