Rotate an Array

Today, I will be talking about rotations. On LeetCode, there is a question that given some array

var head = [1, 2, 3, 4, 5]

and rotate it a number of

var k = 2

times, the final rotation will be

[4, 5, 1, 2, 3]. 

So how shall we approach this problem? We do know that we want to rotate it by removing the last number and inserting it into the front, and keep doing it until we have rotated it k times. The first thing I would do is splice the head array between the array length-k to k

var arr = head.splice(head.length - k, k)

because this will remove the last elements of the array and be inserted into a new array. We then define a new array to be

var newarr = head. 

However, the head array is not the original, but after the splice. If the original head array were

head = [1, 2, 3, 4, 5],

then after the splice, it is

head = [1, 2, 3]. 

Our array is

arr = [5, 4]

and we want to insert these numbers into the front so we simply use a for loop and start at the end and insert into the head array using unshift(). The for loop is

for(var x = arr.length - 1; x >= 0; x--){
newarr.unshift(arr[x])
}
return newarr

The full code is then

function rotate(head, k){
    var arr = head.splice(head.length - k, k)
    var newarr = []
    newarr = head
    for(var x = arr.length - 1; x >= 0; x--){
       newarr.unshift(arr[x])
    }
    return newarr
}

Let’s give it a whirl. Given rotate([1, 2, 3, 4, 5], 2), the answer should be

[4, 5, 1, 2, 3],

and from figure 1, it is.

Fig. 1: Solution for rotate([1, 2, 3, 4, 5], 2).

Let’s try another one: rotate([0, 1, 2], 4). This solution should be

[2, 0, 1]

and figure 2 shows it is indeed.

Fig. 2: Solution for rotate([0, 1, 2], 4).

We can see the code works. If you know of another way to solve it, please tell me and we can discuss it.

References

1. https://leetcode.com/problems/rotate-list/