189. Rotate Array (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].

Note:
Try to come up as many solutions as you can, there are at least 3 different ways to solve this problem.

[show hint]

Hint:

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

Related problem:Reverse Words in a String II

Solution 1: O(n); O(1)

let a= [1,2,3,4,5,6,7]
k = 3.

we have to first reverse the whole array by swapping first element with the last one and so on…
you will get[7,6,5,4,3,2,1]

reverse the elements from 0 to k-1
reverse the elements 7,6,5
you will get [5,6,7,4,3,2,1]

reverse the elements from k to n-1
reverse the elements 4,3,2,1
you will get[5,6,7,1,2,3,4]

    public void rotate(int[] nums, int k) {
        int n = nums.length;
        k = k % n;
        reverse(nums, 0, n - 1);
        reverse(nums,0, k - 1);
        reverse(nums,k, n - 1);
    }

    private void reverse(int[] nums, int l, int r) {
        while (l < r) {
            int temp = nums[l];
            nums[l] = nums[r];
            nums[r] = temp;
            l++;
            r--;
        }
    }