Question

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?

Solution

The solution can be obtained by reversing the entire array, then reversing the first k elements, and reversing the last n-k elements.

class Solution(object):
    def rotate(self, nums, k):
        """
        :type nums: List[int]
        :type k: int
        :rtype: void Do not return anything, modify nums in-place instead.
        """
        length = len(nums)
        if length <= 1 or k == 0 or k % length == 0:
            return

        nums.reverse()
        if k > length:
            k = k % length

        left, right = 0, k - 1
        while right > left:
            nums[left], nums[right] = nums[right], nums[left]
            left += 1
            right -= 1

        left, right = k, length - 1
        while right > left:
            nums[left], nums[right] = nums[right], nums[left]
            left += 1
            right -= 1