Today I have done leetcode's August LeetCoding Challenge with cpp.

August LeetCoding Challenge 11

Description

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.

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

Solution

class Solution {
  int prev;
  bool hasPrev = false;
  
public:
  bool isValidBST(TreeNode* root) {
    if(!root) return true;
    if(!isValidBST(root->left)) return false;
    if(!hasPrev) {
      prev = root->val;
      hasPrev = true;
    } else {
      if(root->val <= prev) return false;
      prev = root->val;
    }
    return isValidBST(root->right);
  }
};

// Accepted
// 80/80 cases passed (24 ms)
// Your runtime beats 30.66 % of cpp submissions
// Your memory usage beats 30.71 % of cpp submissions (21.7 MB)