Add solution 061 [Java]

Author: yanglbme

README.md

@@ -15,7 +15,8 @@ Complete solutions to Leetcode problems, updated daily.
 
 ## Solution List
 
-### Easy
+<detail>
+<summary><b>Easy</b></summary>
 
 | # | Title | Tags |
 |---|---|---|
@@ -40,8 +41,11 @@ Complete solutions to Leetcode problems, updated daily.
 | 703 | [Kth Largest Element in a Stream](https://github.com/doocs/leetcode/tree/master/solution/703.Kth%20Largest%20Element%20in%20a%20Stream) | `Heap` |
 | 876 | [Middle of the Linked List](https://github.com/doocs/leetcode/tree/master/solution/876.Middle%20of%20the%20Linked%20List) | `Linked List` |
 
+</detail>
 
-### Medium
+
+<detail>
+<summary><b>Medium</b></summary>
 
 | # | Title | Tags |
 |---|---|---|
@@ -52,6 +56,7 @@ Complete solutions to Leetcode problems, updated daily.
 | 031 | [Next Permutation](https://github.com/doocs/leetcode/tree/master/solution/031.Next%20Permutation) | `Array` |
 | 046 | [Permutations](https://github.com/doocs/leetcode/tree/master/solution/046.Permutations) | `Backtracking` |
 | 047 | [Permutations II](https://github.com/doocs/leetcode/tree/master/solution/047.Permutations%20II) | `Backtracking` |
+| 061 | [Rotate List](https://github.com/doocs/leetcode/tree/master/solution/061.Rotate%20List) | `Linked List`, `Two Pointers` |
 | 062 | [Unique Paths](https://github.com/doocs/leetcode/tree/master/solution/062.Unique%20Paths) | `Array`, `Dynamic Programming` |
 | 063 | [Unique Paths II](https://github.com/doocs/leetcode/tree/master/solution/063.Unique%20Paths%20II) | `Array`, `Dynamic Programming` |
 | 075 | [Sort Colors](https://github.com/doocs/leetcode/tree/master/solution/075.Sort%20Colors) | `Array`, `Two Pointers`, `Sort` |
@@ -67,8 +72,10 @@ Complete solutions to Leetcode problems, updated daily.
 | 150 | [Evaluate Reverse Polish Notation](https://github.com/doocs/leetcode/tree/master/solution/150.Evaluate%20Reverse%20Polish%20Notation) | `Stack` |
 | 153 | [Find Minimum in Rotated Sorted Array](https://github.com/doocs/leetcode/tree/master/solution/153.Find%20Minimum%20in%20Rotated%20Sorted%20Array) | `Array`, `Binary Search` |
 
+</detail>
 
-### Hard
+<detail>
+<summary><b>Hard</b></summary>
 
 | # | Title | Tags |
 |---|---|---|
@@ -78,6 +85,8 @@ Complete solutions to Leetcode problems, updated daily.
 | 145 | [Binary Tree Postorder Traversal](https://github.com/doocs/leetcode/tree/master/solution/145.Binary%20Tree%20Postorder%20Traversal) | `Stack`, `Tree` |
 | 295 | [Find Median from Data Stream](https://github.com/doocs/leetcode/tree/master/solution/295.Find%20Median%20from%20Data%20Stream) | `Heap`, `Design` |
 
+</detail>
+
 ## Contributions
 I'm looking for long-term contributors/partners to this repo! Send me PRs if you're interested! See the following:
 - Fork this repository to your own GitHub account and then [clone](https://help.github.com/articles/cloning-a-repository/) it to your local device.

solution/061.Rotate List/README.md

@@ -0,0 +1,72 @@
+## 旋转链表
+### 题目描述
+
+给定一个链表，旋转链表，将链表每个节点向右移动 k 个位置，其中 k 是非负数。
+
+**示例 1:**
+```
+输入: 1->2->3->4->5->NULL, k = 2
+输出: 4->5->1->2->3->NULL
+解释:
+向右旋转 1 步: 5->1->2->3->4->NULL
+向右旋转 2 步: 4->5->1->2->3->NULL
+```
+
+**示例 2:**
+```
+输入: 0->1->2->NULL, k = 4
+输出: 2->0->1->NULL
+解释:
+向右旋转 1 步: 2->0->1->NULL
+向右旋转 2 步: 1->2->0->NULL
+向右旋转 3 步: 0->1->2->NULL
+向右旋转 4 步: 2->0->1->NULL
+```
+
+### 解法
+利用双指针`p`,`q`分别指向链表的头部和尾部，题目是右移 k 个位置，右移时，`q`需要指向`q`的前一个位置，似乎不太好做。换种思路，改用左移，右移 k 位相当于左移 len-k 位。循环移位即可。
+
+```java
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    public ListNode rotateRight(ListNode head, int k) {
+        if (head == null) {
+            return head;
+        }
+        int len = 1;
+        ListNode p = head;
+        ListNode q = head;
+        ListNode t = p.next;
+        
+        while (q.next != null) {
+            ++len;
+            q = q.next;
+        }
+        if (len == 1 || k % len == 0) {
+            return head;
+        }
+        
+        k %= len;
+        
+        // 右移 k 个位置，相当于左移 (len-k) 个位置
+        k = len - k;
+        
+        for (int i = 0; i < k; ++i) {
+            q.next = p;
+            p.next = null;
+            q = q.next;
+            p = t;
+            t = p.next;
+        }
+        
+        return p;  
+    }
+}
+```

solution/061.Rotate List/Solution.java

@@ -0,0 +1,42 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    public ListNode rotateRight(ListNode head, int k) {
+        if (head == null) {
+            return head;
+        }
+        int len = 1;
+        ListNode p = head;
+        ListNode q = head;
+        ListNode t = p.next;
+        
+        while (q.next != null) {
+            ++len;
+            q = q.next;
+        }
+        if (len == 1 || k % len == 0) {
+            return head;
+        }
+        
+        k %= len;
+        
+        // 右移 k 个位置，相当于左移 (len-k) 个位置
+        k = len - k;
+        
+        for (int i = 0; i < k; ++i) {
+            q.next = p;
+            p.next = null;
+            q = q.next;
+            p = t;
+            t = p.next;
+        }
+        
+        return p;
+    }
+}