A left rotation operation on an array of size `N`

shifts each of the array’s elements one unit to the left. For example, if two left rotations are performed on array `[1,2,3,4,5]`

, then the array would become `[3,4,5,1,2]`

. Given an array of `N`

integers and a number, `D`

, perform `D`

left rotations on the array. Then print the updated array as a single line of space-separated integers.

The first line contains two space-separated integers denoting the respective values of `N`

(the number of integers) and `D`

(the number of left rotations you must perform). The second line contains `N`

space-separated integers describing the respective elements of the array’s initial state.

```
// Input
5 4
1 2 3 4 5
// Output
5 1 2 3 4
```

Let’s start with the brute-force solution, and we can do this in two different ways. One way is to iterate over the list `D`

times and for each iteration we will modify the value of a new variable, the value for each iteration will be the numbers from position `K`

_(where K is the index of the iteration) to the end of the list, plus the numbers from position zero to `K`

.

```
var newList []int
for i := 1; i <= d; i++ {
newList = append(list[i:], list[0:i]...)
}
```

This will generate a new array with the same size as the original list. A better brute-force approach would be to update the content of the array in-place by setting a new variable only for the first number in the list for each iteration, then replace the value of the array with the numbers from position `one`

to the end of the original list, and finally appending the number from the temporary variable.

```
var temp int
for i := 0; i < d; i++ {
temp = list[0]
list = append(list[1:], temp)
}
```

Good, now the additional memory is simply `O(1)`

. However, we are still doing `O(n)`

with the complexity of the iteration. Let’s see if we can optimize the code even more. We have that for `D=4`

we are returning `A[D:] + A[0:D]`

so it is logical that we can simply move everything from position `D`

to the beginning and the numbers from the beginning to the end.

```
list = append(list[d:], list[0:d]...)
```

Awesome! We have taken both the complexity and memory to `O(1)`

. However, this only works if `D < N`

because otherwise we will raise an index out of range exception as we are trying to access positions that do not exist in the array with the first list selection `A[D:]`

. Let’s see what happens to the array when `D=5`

through `D=10`

.

```
D=5; N[1, 2, 3, 4, 5]
[2 3 4 5 1]
[3 4 5 1 2]
[4 5 1 2 3]
[5 1 2 3 4]
[1 2 3 4 5]
```

```
D=6; N[1, 2, 3, 4, 5]
[2 3 4 5 1]
[3 4 5 1 2]
[4 5 1 2 3]
[5 1 2 3 4]
[1 2 3 4 5]
[2 3 4 5 1]
```

```
D=7; N[1, 2, 3, 4, 5]
[2 3 4 5 1]
[3 4 5 1 2]
[4 5 1 2 3]
[5 1 2 3 4]
[1 2 3 4 5]
[2 3 4 5 1]
[3 4 5 1 2]
```

```
D=8; N[1, 2, 3, 4, 5]
[2 3 4 5 1]
[3 4 5 1 2]
[4 5 1 2 3]
[5 1 2 3 4]
[1 2 3 4 5]
[2 3 4 5 1]
[3 4 5 1 2]
[4 5 1 2 3]
```

```
D=9; N[1, 2, 3, 4, 5]
[2 3 4 5 1]
[3 4 5 1 2]
[4 5 1 2 3]
[5 1 2 3 4]
[1 2 3 4 5]
[2 3 4 5 1]
[3 4 5 1 2]
[4 5 1 2 3]
[5 1 2 3 4]
```

```
D=10; N[1, 2, 3, 4, 5]
[2 3 4 5 1]
[3 4 5 1 2]
[4 5 1 2 3]
[5 1 2 3 4]
[1 2 3 4 5]
[2 3 4 5 1]
[3 4 5 1 2]
[4 5 1 2 3]
[5 1 2 3 4]
[1 2 3 4 5]
```

We notice that for both `D=5`

and `D=10`

the array gets to its original form, so for a list of `N`

numbers if we do `N`

rotations we get the same result as if the rotation was not applied, we can assume that `D=0 if D==N`

, or `D=6`

we have that `D=1 if D==N+1`

, and so on like this:

```
D=5; D=0 if D==N+0
D=6; D=1 if D==N+1
D=7; D=2 if D==N+2
D=8; D=3 if D==N+3
D=9; D=4 if D==N+4
D=10; D=0 if D==N+N
```

We can see a pattern here already, but lets take `D=15`

through `D=20`

:

```
D=15; D=15%N = 0
D=16; D=16%N = 1
D=17; D=17%N = 2
D=18; D=18%N = 3
D=19; D=19%N = 4
D=20; D=20%N = 0
```

Good, so we can reduce any number of rotations greater than the number of items in the list by extracting the modulo, this means we can still keep `O(n)`

no matter how big the number of rotations is.

```
func solution(list []int, d int) []int {
d = d % n /* Keep rotations in range */
return append(list[d:], list[0:d]...)
}
```