add 189

Author: andavid

README.md

@@ -18,7 +18,7 @@
 | 26   | [Remove Duplicates from Sorted Array][026]  |
 | 121  | [Best Time to Buy and Sell Stock][121]      |
 | 122  | [Best Time to Buy and Sell Stock II][122]   |
-| 189  | Rotate Array                                |
+| 189  | [Rotate Array][189]                         |
 
 
 
@@ -29,3 +29,4 @@
 [026]: https://github.com/andavid/leetcode-java/blob/master/note/026/README.md
 [121]: https://github.com/andavid/leetcode-java/blob/master/note/121/README.md
 [122]: https://github.com/andavid/leetcode-java/blob/master/note/122/README.md
+[189]: https://github.com/andavid/leetcode-java/blob/master/note/189/README.md

note/189/README.md

@@ -0,0 +1,42 @@
+# [Rotate Array][title]
+
+## Description
+
+Rotate an array of n elements to the right by k steps.
+
+For example, with n = 7 and k = 3, the array `[1,2,3,4,5,6,7]` is rotated to `[5,6,7,1,2,3,4]`.
+
+**Note:**  
+Try to come up as many solutions as you can, there are at least 3 different ways to solve this problem.
+
+## 思路
+三步反转法
+
+(X<sup>T</sup>Y<sup>T</sup>)<sup>T</sup> = YX
+
+这里需要特别注意，向右移动的步数 k 有可能大于数组的长度，因此要做个取模处理。
+
+## [完整代码][src]
+
+```java
+class Solution {
+    public void rotate(int[] nums, int k) {
+        int n = nums.length;
+        k %= n;
+        reverse(nums, 0, n - k - 1);
+        reverse(nums, n - k, n - 1);
+        reverse(nums, 0, n - 1);
+    }
+    
+    private void reverse(int[] nums, int from, int to) {
+        while (from < to) {
+            int temp = nums[from];
+            nums[from++] = nums[to];
+            nums[to--] = temp;
+        }
+    }
+}
+```
+
+[title]: https://leetcode.com/problems/rotate-array
+[src]: https://github.com/andavid/leetcode-java/blob/master/src/com/andavid/leetcode/_189/Solution.java

src/com/andavid/leetcode/_189/Solution.java

@@ -0,0 +1,29 @@
+package com.andavid.leetcode._189;
+
+public class Solution {
+    public void rotate(int[] nums, int k) {
+        int n = nums.length;
+        k %= n;
+        reverse(nums, 0, n - k - 1);
+        reverse(nums, n - k, n - 1);
+        reverse(nums, 0, n - 1);
+    }
+
+    private void reverse(int[] nums, int from, int to) {
+        while (from < to) {
+            int temp = nums[from];
+            nums[from++] = nums[to];
+            nums[to--] = temp;
+        }
+    }
+
+    public static void main(String[] args) {
+        Solution solution = new Solution();
+        int[] nums = {1, 2, 3, 4, 5, 6, 7};
+        solution.rotate(nums, 3);
+        int len = nums.length;
+        for (int i = 0; i < len; i++) {
+            System.out.print(nums[i] + (i == len - 1 ? "" : ", "));
+        }
+    }
+}