Coding

Left Rotate Array by K Places

Asked at

Oracle

Visa

1Give yourself 5 minutes

2Answer out loud, not in your head

3Then compare with the answer below

The problem

Given an integer array nums and a non-negative integer k, rotate the array to the left by k steps.

Input: nums = [1, 2, 3, 4, 5, 6], k = 2 Output: nums = [3, 4, 5, 6, 1, 2] Explanation: rotate 1 step to the left: [2, 3, 4, 5, 6, 1] rotate 2 steps to the left: [3, 4, 5, 6, 1, 2]

Input: nums = [3, 4, 1, 5, 3, -5], k = 8 Output: nums = [1, 5, 3, -5, 3, 4] Explanation: rotate 1 step to the left: [4, 1, 5, 3, -5, 3] rotate 2 steps to the left: [1, 5, 3, -5, 3, 4] rotate 3 steps to the left: [5, 3, -5, 3, 4, 1] rotate 4 steps to the left: [3, -5, 3, 4, 1, 5] rotate 5 steps to the left: [-5, 3, 4, 1, 5, 3] rotate 6 steps to the left: [3, 4, 1, 5, 3, -5] rotate 7 steps to the left: [4, 1, 5, 3, -5, 3] rotate 8 steps to the left: [1, 5, 3, -5, 3, 4]

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

1 <= nums.length <= 105
-104 <= nums[i] <= 104
0 <= k <= 105

cpp

class Solution {
public:
    void rotateArray(vector<int>& nums, int k) {

    }
};

java

class Solution {
    public void rotateArray(int[] nums, int k) {
        
    }
}

python

class Solution:
    def rotateArray(self, nums, k: int) -> None:

javascript

class Solution {
    rotateArray(nums, k) {

    }
}

csharp

public class Solution {
    public void rotateArray(List<int> nums, int k) {

    }
}

func rotateArray(nums []int, k int) {
    //your code goes here
}

Stuck? Show a way to structure it+

01Normalize k with `k %= n`.
02Reverse the first k elements.
03Reverse the remaining n-k elements.
04Reverse the entire array.

Reference answer

A left rotation by k can be performed in place with three reversals. First normalize k %= n. Reverse [0,k-1], reverse [k,n-1], then reverse the whole array. For abcdef and k=2, this produces ba|fed, then cdefab. The first two reversals reverse each logical block, and the final reversal restores the internal order of each block while exchanging their positions. Each element is swapped a constant number of times, giving O(n) time and O(1) extra space. Guard the empty-array case before taking modulo. A temporary-array implementation is also O(n) but does not meet a strict constant-extra-space preference. If the API permits a negative k, normalize it into [0,n) before applying the same method.

Then expect these follow-ups

How would you rotate right by k?
Tests: transformation reasoning
How would you rotate a linked list?
Tests: pointer manipulation

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