98. Validate Binary Search Tree | LeetCode Solution

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

/**

 * 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);

  }

};