[Leetcode] 98. Validate Binary Search Tree - 문제풀이

Description

주어진 이진트리가 유효한 이진탐색트리인지 여부를 반환하는 문제입니다.

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, 10^4].
• -2^31 <= Node.val <= 2^31 - 1

Solution 1. Recursion(재귀)

public boolean isValidBST(TreeNode root, Integer min, Integer max) {
    //1. Recursion - DFS
    if(root == null) return true;
    if(min != null && root.val <= min) return false;
    if(max != null && root.val >= max) return false;

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

깊이우선탐색 재귀호출을 통하여 노드를 탐색해 나가며 유효성을 체크합니다. 왼쪽 노드는 root노드보다 클 수 없고 오른쪽노드는 root노드보다 작을 수 없기때문에 최소,최대값을 유지해나가며 재귀호출을 실행합니다.

Reference

Validate Binary Search Tree - LeetCode