98. Validate Binary Search Tree

Problem Statement

Medium

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.

Example 1:

Input: root = [2,1,3]
Output: true

Example 2:

Input: root = [5,1,4,null,null,3,6]
Output: false
Explanation: The root node's value is 5 but its right child's value is 4.

Constraints:

The number of nodes in the tree is in the range [1, 10⁴].
-2³¹ <= Node.val <= 2³¹ - 1

C++

Source file

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    // Simple Soln : Do InOrder Traversal -> must be strictly increasing
    long int maxm = LONG_MIN;
    bool valid = true;
    
    bool isValidBST(TreeNode* root) {
        if(!valid) return valid;
        if(!root) return true;
        auto rootVal = root->val;
        auto lt = root->left;
        auto rt = root->right;
        if(lt)
            isValidBST(lt);
        valid &= (maxm<rootVal);
        maxm = max(maxm, (long int)rootVal);
        if(rt)
            isValidBST(rt);
        return valid;
    }
};