LeetCode - Verify Binary Search Tree

Given a binary tree, determine whether it is a valid binary search tree.

A binary search tree has the following characteristics:

• The left subtree of a node contains only numbers less than the current node.
• The right subtree of a node contains only numbers greater than the current node.
• All left and right subtrees must themselves be binary search trees.

Example 1:

enter:
    2
   / \
  1   3
output: true

Example 2:

enter:
    5
   / \
  1   4
     / \
    3   6
output: false
Explanation: The input is: [5,1,4,null,null,3,6].
     The root node has a value of 5, but its right child has a value of 4.

---

solution:

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     bool isValidBST(TreeNode* root) {
13         return validateRange(root, INT32_MIN, INT32_MAX);
14     }
15 private:
16     bool validateRange(TreeNode* root, int min, int max)
17     {
18         if (root == nullptr)
19             return true;
20     
21         if (root->val == INT32_MIN && root->left != nullptr)
22             return false;
23     
24         if (root->val == INT32_MAX && root->right != nullptr)
25             return false;
26     
27         if (root->val < min || root->val > max)
28             return false;
29     
30         bool leftFlag  = validateRange(root->left, min, root->val-1);
31         bool rightFlag = validateRange(root->right, root->val+1, max);
32     
33         return leftFlag && rightFlag;
34     }
35 };