[LeetCode] 235. Lowest Common Ancestor of a Binary Search Tree - 문제풀이

Description

이진검색트리에서 주어진 두 노드의 가장 최하위 공통 조상 LCA를 찾아 반환하는 문제입니다.

Given a binary search tree (BST), find the lowest common ancestor (LCA) of two given nodes in the BST.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”

Example 1:

Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 8
Output: 6
Explanation: The LCA of nodes 2 and 8 is 6.

Example 2:

Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 4
Output: 2
Explanation: The LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.

Example 3:

Input: root = [2,1], p = 2, q = 1
Output: 2

Constraints:

• The number of nodes in the tree is in the range [2, 10^5].
• -10^9 <= Node.val <= 10^9
• All Node.val are unique.
• p != q
• p and q will exist in the BST.

Solution 1. DFS - Recursion(재귀)

public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
    // 1. Recursion
    if(root.val > p.val && root.val > q.val){
        return lowestCommonAncestor(root.left, p, q);
    }else if(root.val < p.val && root.val < q.val){
        return lowestCommonAncestor(root.right, p, q);
    }else{
        return root;
    }
}

root가 주어진 두노드보다 클 경우 작은 값인 왼쪽 노드를 재귀로 탐색, root가 두노드보다 작을 경우는 더 큰값을 탐색하기 위해 오른쪽 노드를 탐색하고 그외에 값이 같거나 사이에 있을 경우므로 root를 반환하는 재귀 호출을 해줍니다.

Reference

Lowest Common Ancestor of a Binary Search Tree - LeetCode