LeetCode-in-Java

189. Rotate Array

Medium

Given an array, rotate the array to the right by k steps, where k is non-negative.

Example 1:

Input: nums = [1,2,3,4,5,6,7], k = 3

Output: [5,6,7,1,2,3,4]

Explanation:

rotate 1 steps to the right: [7,1,2,3,4,5,6]
rotate 2 steps to the right: [6,7,1,2,3,4,5]
rotate 3 steps to the right: [5,6,7,1,2,3,4] 

Example 2:

Input: nums = [-1,-100,3,99], k = 2

Output: [3,99,-1,-100]

Explanation:

rotate 1 steps to the right: [99,-1,-100,3]
rotate 2 steps to the right: [3,99,-1,-100] 

Constraints:

• 1 <= nums.length <= 105
• -231 <= nums[i] <= 231 - 1
• 0 <= k <= 105

Follow up:

• Try to come up with as many solutions as you can. There are at least three different ways to solve this problem.
• Could you do it in-place with O(1) extra space?

Solution

public class Solution {
    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--;
        }
    }

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

Time Complexity (Big O Time):

The time complexity of this program is O(n), where n is the length of the input array nums. Here’s why:

1. The reverse method has a while loop that iterates from l to r, where l and r are indices within the array. The number of iterations in this loop is proportional to the size of the subarray being reversed.

2. In the rotate method, there are three calls to the reverse method:

   • The first reverse call reverses the subarray from index 0 to t - 1, where t is calculated as n - (k % n).
   • The second reverse call reverses the subarray from index t to n - 1.
   • The third reverse call reverses the entire array from index 0 to n - 1.

All three reverse calls have time complexity proportional to the size of the subarray being reversed, and they operate on non-overlapping subarrays. Therefore, the overall time complexity is O(n).

Space Complexity (Big O Space):

The space complexity of this program is O(1), indicating that it uses a constant amount of additional memory regardless of the size of the input array. The program performs the rotation in-place, and the space used for variables and computations remains constant.

In summary, the time complexity is O(n), where n is the length of the input array, and the space complexity is O(1), indicating constant space usage.

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