add: 环形链表

README.md

@@ -333,9 +333,15 @@ LeetCode 与 LintCode 解题记录。此为个人练习仓库，代码中对重
 - [合并两个有序链表](src/list/merge-two-sorted-lists.js)
 
   - LeetCode 21. 合并两个有序链表 https://leetcode-cn.com/problems/merge-two-sorted-lists/
+  - LeetCode 面试题25. 合并两个排序的链表 https://leetcode-cn.com/problems/he-bing-liang-ge-pai-xu-de-lian-biao-lcof
   - LintCode 165. 合并两个排序链表 https://www.lintcode.com/problem/merge-two-sorted-lists/description
 
 - [链表排序](src/list/sort-list.js)
 
   - LeetCode 148. 排序链表 https://leetcode-cn.com/problems/sort-list/
   - LintCode 98. 链表排序 https://www.lintcode.com/problem/sort-list/description
+
+- [环形链表](src/list/linked-list-cycle.js)
+
+  - LeetCode 141. 环形链表 https://leetcode-cn.com/problems/linked-list-cycle/
+  - LintCode 102. 带环链表 https://www.lintcode.com/problem/linked-list-cycle/description

src/list/linked-list-cycle.js

@@ -0,0 +1,28 @@
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val) {
+ *     this.val = val;
+ *     this.next = null;
+ * }
+ */
+
+/**
+ * @param {ListNode} head
+ * @return {boolean}
+ */
+export const hasCycle = function (head) {
+  if (!head || !head.next) return false
+
+  // 双指针解法
+  let slow = head
+  let fast = head.next
+
+  while (true) {
+    if (!fast || !fast.next) return false
+    else if (fast.next === slow || fast === slow) return true
+    else {
+      fast = fast.next.next
+      slow = slow.next
+    }
+  }
+}

test/list/linked-list-cycle.test.js

@@ -0,0 +1,24 @@
+import { hasCycle } from '../../src/list/linked-list-cycle'
+
+function ListNode (val) {
+  this.val = val
+  this.next = null
+}
+
+const arrToList = (arr) => {
+  const head = new ListNode(arr[0])
+  let current = head
+  for (let n = 1, len = arr.length; n < len; n++) {
+    current.next = new ListNode(arr[n])
+    current = current.next
+  }
+
+  return head
+}
+
+test('环形链表', () => {
+  const list = arrToList([1, 2, 3])
+  list.next.next.next = list
+  expect(hasCycle(list)).toBe(true)
+  expect(hasCycle(arrToList([1, 2, 3]))).toBe(false)
+})