189 - rotate array

题目
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].
Could you do it in-place with O(1) extra space?

方案1：

首先分别翻转前n-k个元素组和后k个元素组，再整体翻转一次；

注意：题目没限制k一定小于n，所以要用k=k%n;来防止越界。

该方案空间复杂度O(1),但是时间复杂度线性，代码如下：

class Solution {public:    void rotate(vector<int>& nums, int k) {        int n = nums.size();        k = k%n;        for( int i=0,j=n-k-1; i<j; ++i,--j){            int tmp = nums[i];            nums[i] = nums[j];            nums[j] = tmp;        }        for( int i=n-k,j=n-1; i<j; ++i,--j){            int tmp = nums[i];            nums[i] = nums[j];            nums[j] = tmp;        }        for( int i=0,j=n-1; i<j; ++i,--j){            int tmp = nums[i];            nums[i] = nums[j];            nums[j] = tmp;        }    }};