g0001_0100.s0098_validate_binary_search_tree.Solution Maven / Gradle / Ivy

Go to download

Show more of this group Show more artifacts with this name
Show all versions of leetcode-in-java21 Show documentation

Java-based LeetCode algorithm problem solutions, regularly updated

There is a newer version: 1.38

package g0001_0100.s0098_validate_binary_search_tree;

// #Medium #Top_100_Liked_Questions #Top_Interview_Questions #Depth_First_Search #Tree #Binary_Tree
// #Binary_Search_Tree #Data_Structure_I_Day_14_Tree #Level_1_Day_8_Binary_Search_Tree
// #Udemy_Tree_Stack_Queue #Big_O_Time_O(N)_Space_O(log(N))
// #2022_06_21_Time_0_ms_(100.00%)_Space_43.4_MB_(72.88%)

import com_github_leetcode.TreeNode;

/*
 * 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;
 *     }
 * }
 */
/**
 * 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:**
 *
 * ![](https://assets.leetcode.com/uploads/2020/12/01/tree1.jpg)
 *
 * **Input:** root = [2,1,3]
 *
 * **Output:** true 
 *
 * **Example 2:**
 *
 * ![](https://assets.leetcode.com/uploads/2020/12/01/tree2.jpg)
 *
 * **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
**/
public class Solution {
    public boolean isValidBST(TreeNode root) {
        return solve(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }

    // we will send a valid range and check whether the root lies in the range
    // and update the range for the subtrees
    private boolean solve(TreeNode root, long left, long right) {
        if (root == null) {
            return true;
        }
        if (root.val <= left || root.val >= right) {
            return false;
        }
        return solve(root.left, left, root.val) && solve(root.right, root.val, right);
    }
}