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
-
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
Approach - Recursion
- It should be strictly less and greater so equal values not allowed
- we go left so we update the max to current value
Time: O(n) Space: O(n)
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public boolean isValidBST(TreeNode root) {
return valid(root, Long.MIN_VALUE, Long.MAX_VALUE);
}
public boolean valid(TreeNode n, long min, long max) {
if (n == null) return true;
if (n.val <= min || n.val >= max) return false;
return valid(n.left, min, n.val) && valid(n.right, n.val, max);
}
}