98. Validate Binary Search Tree | LeetCode Solution

 98. Validate Binary Search Tree | LeetCode Solution

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

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

/**
 * 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:
  int rootVal;

  bool helper(TreeNode *root, TreeNode *minNode, TreeNode *maxNode)
  {

    if (!root)
      return true;
    if (minNode && root->val <= minNode->val || maxNode && root->val >= maxNode->val)
      return false;
    return helper(root->left, minNode, root) && helper(root->right, root, maxNode);
  }

  bool isValidBST(TreeNode *root)
  {

    return helper(root, NULL, NULL);
  }
};

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