hackerrank圆形数组旋转执行时错误

Question

I did solved a hackerrank problem which is "Circular Array Rotation" using Go when i run the code it gives me a correct response and when i try to submit the code all the test passes except one and it says Runtime Error

I tried the failing test localy and it passes my code is

    package main
    import "fmt"

    func CircularArrayRotation() {
        var n, k, q int

        fmt.Scanf("%d%d%d", &n, &k, &q)
        a := make([]int, n)
        for i := range a {
            fmt.Scanf("%d", &a[i])
        }

        var i int
        for t:=0; t<q; t++  {

            fmt.Scanf("%d", &i)
            j := (i - k)
            if j<0 {
                j+=n
            }

            fmt.Println(a[j])
        }

    }

func main() {
 //Enter your code here. Read input from STDIN. Print output to STDOUT
    CircularArrayRotation()
}

Answer 1

For those that want more information you can look here: https://www.hackerrank.com/challenges/circular-array-rotation/problem

The Input for his failing case is this and the Expected Output is this

Your run time error is this:

panic: runtime error: index out of range

goroutine 1 [running]: main.CircularArrayRotation() solution.go:22
+0x349 main.main() solution.go:29 +0x20

So your issue is on line 22, where your index is out of range: fmt.Println(a[j])

This happens because your code currently cannot handle multiple rotations, so in your code you end up executing the following:

fmt.Println(a[-99477])

This happens when i is 8 and k is 100000

Imagine you had this input:

n = 3
k = 10
q = 1
i = 2

When you perform i - k we get -8, we then try to add n which gives us -5 (-8 + 3), then we try to access an index that does not exist. a[-5]

If you wish to fix this issue you can do the below (inside spoiler in case you want to work this out yourself):

put this above your i - k
k = k % n

The reason this fixes your code is:

It works out how many rotations are left after we've fully looped x times. That way we don't have to worry about multiple rotations.

--
As a side note for this challenge there's some interesting stuff you can do with slices for rotating an array using some of the stuff covered here: https://tour.golang.org/moretypes/10