189. Rotate Array

• python3

Problem

Given an integer array nums, 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?

Pesudocode

1. Reverse the entire array.
2. Reverse the portion of the array from index 0 to k-1.
3. Reverse the portion of the array from index k to the end of the array.

Code

class Solution:
    def rotate(self, nums: List[int], k: int) -> None:
        """
        Do not return anything, modify nums in-place instead.
        """
        k = k % len(nums)

        # 1. reverse
        nums.reverse()
        
        # 2. reverse first part
        left = 0
        right = k-1
        while left < right:
            nums[left], nums[right] = nums[right], nums[left]
            left+=1
            right-=1

        # 3. reverse second part
        left = k
        right = len(nums) - 1
        while left < right:
            nums[left], nums[right] = nums[right], nums[left]
            left+=1
            right-=1

Result

• Time Complexity : O(n)
• Space Complexity : O(1)