solve(w09): 141. Linked List Cycle

Author: seungriyou

linked-list-cycle/seungriyou.py

@@ -0,0 +1,55 @@
+# https://leetcode.com/problems/linked-list-cycle/
+
+from typing import Optional
+
+# Definition for singly-linked list.
+class ListNode:
+    def __init__(self, x):
+        self.val = x
+        self.next = None
+
+class Solution:
+    def hasCycle_on(self, head: Optional[ListNode]) -> bool:
+        """
+        [Complexity]
+            - TC: O(n)
+            - SC: O(n)
+
+        [Approach]
+            hash table로 visited 노드를 기록한다.
+        """
+        curr = head
+        visited = set()
+
+        while curr:
+            if curr in visited:
+                return True
+
+            visited.add(curr)
+            curr = curr.next
+
+        return False
+
+    def hasCycle(self, head: Optional[ListNode]) -> bool:
+        """
+        [Complexity]
+            - TC: O(n)
+            - SC: O(1)
+
+        [Approach]
+            Floyd’s Cycle Detection 알고리즘을 사용한다.
+            linked list에서 slow (1칸씩 전진) & fast (2칸씩 전진) runner를 이용하면,
+                - slow와 fast가 만난다면 cyclic
+                - 만나지 않은 채로 fast가 끝에 도달하면 not cyclic
+            이다.
+        """
+        slow = fast = head
+
+        while fast and fast.next:
+            slow = slow.next
+            fast = fast.next.next
+
+            if slow == fast:
+                return True
+
+        return False