[LeetCode] 236 | Lowest Common Ancestor of a Binary Tree | Python

---

문제 링크: https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree/

---

문제

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

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).”

---

조건

• 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 tree.

---

내 풀이

Tree문제는 보통 recursive로 푼다.

각각 root의 왼쪽과 오른쪽 node를 탐색했을 때, 왼쪽에 하나, 오른쪽에 하나(if r and l)가 있다면 root를 리턴한다.

만약 오른쪽에만 있다면(elif r) 오른쪽 노드를 리턴하고, 왼쪽에만 있다면(else) 왼쪽 노드를 리턴한다.

---

코드

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def lowestCommonAncestor(self, root: 'TreeNode', p: 'TreeNode', q: 'TreeNode') -> 'TreeNode':
        if root:
            if root == p or root == q:
                return root
            
            l = self.lowestCommonAncestor(root.left, p, q)
            r = self.lowestCommonAncestor(root.right, p, q)
            
            if r and l:
                return root
            elif r:
                return r
            else:
                return l