Linked List Cycle

Author: w2xi

README.md

@@ -12,6 +12,7 @@
 |28|[实现 strStr()](https://leetcode-cn.com/problems/implement-strstr/)|[JavaScript](./algorithms/implement-strstr.js)|Easy|
 |66|[加一](https://leetcode-cn.com/problems/plus-one/)|[JavaScript](./algorithms/plus-one.js)|Easy|
 |136|[只出现一次的数字](https://leetcode-cn.com/problems/single-number/)|[JavaScript](./algorithms/single-number.js)|Easy|
+|141|[环形链表](https://leetcode-cn.com/problems/linked-list-cycle/)|[JavaScript](./algorithms/linked-list-cycle.js)|Easy|
 |189|[轮转数组](https://leetcode-cn.com/problems/rotate-array/)|[JavaScript](./algorithms/rotate-array.js)|Medium|
 |206|[反转链表](https://leetcode-cn.com/problems/reverse-linked-list/)|[JavaScript](./algorithms/reverse-linked-list.js)|Easy|
 |344|[反转字符串](https://leetcode-cn.com/problems/reverse-string/)|[JavaScript](./algorithms/reverse-string.js)|Easy|

algorithms/linked-list-cycle.js

@@ -0,0 +1,62 @@
+// 方法一：集合(Set)
+
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val) {
+ *     this.val = val;
+ *     this.next = null;
+ * }
+ */
+
+/**
+ * @param {ListNode} head
+ * @return {boolean}
+ */
+var hasCycle = function (head) {
+  let curr = head;
+  let s = new Set();
+
+  while (curr) {
+    if (s.has(curr)) {
+      return true;
+    }
+    s.add(curr);
+    curr = curr.next;
+  }
+
+  return false;
+};
+
+// 方法二：快慢指针
+// Reference: https://leetcode-cn.com/leetbook/read/top-interview-questions-easy/xnwzei/
+
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val) {
+ *     this.val = val;
+ *     this.next = null;
+ * }
+ */
+
+/**
+ * @param {ListNode} head
+ * @return {boolean}
+ */
+var hasCycle = function (head) {
+  // 快慢指针
+  let fast = head,
+    slow = head;
+
+  while (fast && fast.next) {
+    // 慢指针每次走一步
+    slow = slow.next;
+    // 快指针每次走两步
+    fast = fast.next.next;
+    // 如果相遇，则说明有环
+    if (fast === slow) {
+      return true;
+    }
+  }
+
+  return false;
+};