/***189. Rotate Array  
Difficulty: Easy
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].
思路： The basic idea is that, for example, nums = [1,2,3,4,5,6,7] and k = 3, first we reverse [1,2,3,4], it becomes[4,3,2,1]; then we reverse[5,6,7], it becomes[7,6,5], finally we reverse the array as a whole, it becomes[4,3,2,1,7,6,5] ---> [5,6,7,1,2,3,4].
Reverse is done by using two pointers, one point at the head and the other point at the tail, after switch these two, these two pointers move one position towards the middle.
*/
 public void rotate(int[] nums, int k) {
        if (nums == null || nums.length < 2) {
            return ;
        }
        k = k % nums.length;
        reverse(nums, 0, nums.length - k - 1);
        reverse(nums, nums.length - k, nums.length - 1);
        reverse(nums, 0, nums.length -1);    
        
    }
    
    // use two pointers 
    private void reverse(int[] nums, int begin, int end) {
        while (begin < end) {
            int temp = nums[begin];
            nums[begin] = nums[end];
            nums[end] = temp;
            begin++;
            end--;
        }
    }**