Problem: Validate Binary Search Tree

Question

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

Assume a 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:

    2
   / \
  1   3
Binary tree [2,1,3], return true.

Example 2:

    1
   / \
  2   3
Binary tree [1,2,3], return false.

Solution

TODO

Code

class Solution {
    public boolean isValidBST(TreeNode root) {
        return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }

    public boolean isValidBST(TreeNode node, long min, long max) {
        return node == null || node.val > min && node.val < max
            && isValidBST(node.left, min, node.val)
            && isValidBST(node.right, node.val, max);
    }
}

Performance