Add solution 141 [Java]

Author: yanglbme

README.md

@@ -31,6 +31,7 @@ Complete [solutions](https://github.com/doocs/leetcode/tree/master/solution) to
 | 070 | [Climbing Stairs](https://github.com/doocs/leetcode/tree/master/solution/070.Climbing%20Stairs) | `Dynamic Programming` |
 | 083 | [Remove Duplicates from Sorted List](https://github.com/doocs/leetcode/tree/master/solution/083.Remove%20Duplicates%20from%20Sorted%20List) | `Linked List` |
 | 136 | [Single Number](https://github.com/doocs/leetcode/tree/master/solution/136.Single%20Number) | `Hash Table`, `Bit Manipulation` |
+| 141 | [Linked List Cycle](https://github.com/doocs/leetcode/tree/master/solution/141.Linked%20List%20Cycle) | `Linked List`, `Two Pointers` |
 | 189 | [Rotate Array](https://github.com/doocs/leetcode/tree/master/solution/189.Rotate%20Array) | `Array` |
 | 198 | [House Robber](https://github.com/doocs/leetcode/tree/master/solution/198.House%20Robber) | `Dynamic Programming` |
 | 203 | [Remove Linked List Elements](https://github.com/doocs/leetcode/tree/master/solution/203.Remove%20Linked%20List%20Elements) | `Linked List` |

solution/141.Linked List Cycle/README.md

@@ -0,0 +1,39 @@
+## 环形链表
+### 题目描述
+
+给定一个链表，判断链表中是否有环。
+
+**进阶：**
+你能否不使用额外空间解决此题？
+
+### 解法
+
+利用快慢指针，若快指针为 null，则不存在环，若快慢指针相遇，则存在环。
+
+```java
+/**
+ * Definition for singly-linked list.
+ * class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) {
+ *         val = x;
+ *         next = null;
+ *     }
+ * }
+ */
+public class Solution {
+    public boolean hasCycle(ListNode head) {
+        ListNode slow = head;
+        ListNode fast = head;
+        while (fast != null && fast.next != null) {
+            slow = slow.next;
+            fast = fast.next.next;
+            if (slow == fast) {
+                return true;
+            }
+        }
+        return false;
+    }
+}
+```

solution/141.Linked List Cycle/Solution.java

@@ -0,0 +1,25 @@
+/**
+ * Definition for singly-linked list.
+ * class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) {
+ *         val = x;
+ *         next = null;
+ *     }
+ * }
+ */
+public class Solution {
+    public boolean hasCycle(ListNode head) {
+        ListNode slow = head;
+        ListNode fast = head;
+        while (fast != null && fast.next != null) {
+            slow = slow.next;
+            fast = fast.next.next;
+            if (slow == fast) {
+                return true;
+            }
+        }
+        return false;
+    }
+}