Question

Formatted question description: https://leetcode.ca/all/189.html

 189. Rotate Array

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

 Example 1:

 Input: [1,2,3,4,5,6,7] and 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: [-1,-100,3,99] and 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]

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

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

 @tag-array

Algorithm

O(1) extra space.

Similar to the method of flipping characters, the idea is to flip the first n-k numbers first, then flip the last k numbers, and finally flip the entire array.

[1,2,3,4,5,6,7] => [4,3,2,1,5,6,7]

[4,3,2,1,5,6,7] => [4,3,2,1,7,6,5]

[4,3,2,1,7,6,5] => [5,6,7,1,2,3,4]

Code

Java

import org.apache.commons.lang3.ArrayUtils;

public class Rotate_Array {

    class Solution {
        public void rotate(int[] nums, int k) {
            if (nums.length == 0 || (k %= nums.length) == 0) {
                return;
            }

            int n = nums.length;

            // [1,2,3,4,5,6,7] => [4,3,2,1,5,6,7]
            ArrayUtils.reverse(nums, 0, n - k); // @note: org.apache.commons.lang3.ArrayUtils

            // [4,3,2,1,5,6,7] => [4,3,2,1,7,6,5]
            ArrayUtils.reverse(nums, n - k, nums.length);

            // [4,3,2,1,7,6,5] => [5,6,7,1,2,3,4]
            ArrayUtils.reverse(nums, 0, nums.length);
        }
    }

    class Solution_kTimesAllItemsMoving {
        public void rotate(int[] nums, int k) {

            while ( k > 0) {
                k--;

                int tmp = nums[nums.length - 1]; // last one
                for (int i = nums.length - 1; i >= 1; i--) {
                    nums[i] = nums[i - 1];
                }

                nums[0] = tmp;
            }
        }
    }
}

All Problems

All Solutions