Q23_validate_binary_search_tree.cpp

// 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.

#include<bits/stdc++.h>
using namespace std;

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) {}
};

// Important question

class Solution {
private:
    bool validateBST(TreeNode* root, long mini, long maxi) {
        if(!root) return true;
        if(root->val >= maxi || root->val <= mini) return false;

        bool left = validateBST(root->left, mini, root->val);
        bool right = validateBST(root->right, root->val, maxi);

        return left && right;
    }
public:
    bool isValidBST(TreeNode* root) {
        return validateBST(root, LONG_MIN, LONG_MAX);
    }
};

// Time Complexity  : O(n) 
// Space Complexity : O(h)
// where h is the height of the tree