LeetCode-in-Dart
98. Validate Binary Search Tree
Medium
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
/**
* Definition for a binary tree node.
* class TreeNode {
* int val;
* TreeNode? left;
* TreeNode? right;
* TreeNode([this.val = 0, this.left, this.right]);
* }
*/
class Solution {
bool isValidBST(TreeNode? root) {
return _isValidBSTHelper(root, null, null);
}
bool _isValidBSTHelper(TreeNode? node, int? min, int? max) {
if (node == null) {
return true;
}
if ((min != null && node.val <= min) || (max != null && node.val >= max)) {
return false;
}
return _isValidBSTHelper(node.left, min, node.val) &&
_isValidBSTHelper(node.right, node.val, max);
}
}