Solution 2

Author: obzva

lowest-common-ancestor-of-a-binary-search-tree/flynn.go

@@ -0,0 +1,35 @@
+/*
+풀이
+- common ancestor라는 조건에 맞는 node를 찾아냅니다
+  for common ancestor C, left subtree of C includes p (p.Val <= C.Val)
+                     and right subtree of C includes q (C.Val <= q.Val)
+  이러한 조건을 만족하는 common ancestor는 BST에 하나만 존재합니다
+  따라서 common ancestor를 찾으면 그게 곧 lowest common ancestor입니다
+Big O
+- N: 노드의 개수
+- H: 트리의 높이 (avg: logN, worst: N)
+- Time complexity: O(H)
+- Space complexity: O(H)
+*/
+
+/**
+ * Definition for a binary tree node.
+ * type TreeNode struct {
+ *     Val   int
+ *     Left  *TreeNode
+ *     Right *TreeNode
+ * }
+ */
+
+ func lowestCommonAncestor(root, p, q *TreeNode) *TreeNode {
+	if p.Val > q.Val { // p < q가 되도록 함
+		return lowestCommonAncestor(root, q, p)
+	}
+	if p.Val <= root.Val && root.Val <= q.Val { // common ancestor를 찾음
+		return root
+	} else if q.Val < root.Val { // left subtree 탐색
+		return lowestCommonAncestor(root.Left, p, q)
+	} else { // right subtree 탐색
+		return lowestCommonAncestor(root.Right, p, q)
+	}
+}