98.validate-binary-search-tree.java

/*
 * @lc app=leetcode id=98 lang=java
 *
 * [98] Validate Binary Search Tree
 *
 * https://leetcode.com/problems/validate-binary-search-tree/description/
 *
 * algorithms
 * Medium (25.24%)
 * Total Accepted:    383.7K
 * Total Submissions: 1.5M
 * Testcase Example:  '[2,1,3]'
 *
 * 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:
 *
 *
 * Input:
 * ⁠   2
 * ⁠  / \
 * ⁠ 1   3
 * Output: true
 *
 *
 * Example 2:
 *
 *
 * ⁠   5
 * ⁠  / \
 * ⁠ 1   4
 * / \
 * 3   6
 * Output: false
 * Explanation: The input is: [5,1,4,null,null,3,6]. The root node's
 * value
 * is 5 but its right child's value is 4.
 *
 *
 */
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
  public boolean isValidBST(TreeNode root) {
    return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE);
  }

  public boolean isValidBST(TreeNode root, long min, long max) {
    if (root == null) {
      return true;
    }

    return (min < root.val) && (root.val < max)
      && isValidBST(root.left, min, root.val)
      && isValidBST(root.right, root.val, max);
  }
}