Go语言创建排列

Question

Today I tried to do my first steps with go. I tried to write a function, which creates all permutations of a given list. First I failed completely, so I tried to write the function with python and translate it step by step to go:

python:

def get_permutations(elements):
    permutations = []
    if len(elements) == 1:
        return [elements]
    for i in range(len(elements)):
        for perm in get_permutations(elements[0:i] +  elements[i+1:]):
            permutations.append([elements[i]] + perm)
    return permutations

print(get_permutations([1,2,3]))

go:

func getPermutations(elements []int) [][]int {
    permutations := [][]int{}
    if len(elements) == 1 {
        permutations = [][]int{elements}
        return permutations
    }
    for i := range elements {
        for _, perm := range getPermutations(append(elements[0:i], elements[i+1:]...)) {
            permutations = append(permutations, append([]int{elements[i]}, perm...))
        }
    }
    return permutations
}

func main() {
    x := getPermutations([]int{1, 2, 3})
    fmt.Print(x)
}

While the python version creates this ouput:

[[1, 2, 3], [1, 3, 2], [2, 1, 3], [2, 3, 1], [3, 1, 2], [3, 2, 1]]

The go version creates this one:

[[3 3 3] [3 3 3] [3 3 3] [3 3 3] [3 3 3] [3 3 3]]

I really someone can help me. I really would like to know, what I've done wrong in the go code

Answer 1

The getPermutations function is mutating the original elements slice each iteration. You need to make a copy of that slice before you modify it.

func getPermutations(elements []int) [][]int {
    permutations := [][]int{}
    if len(elements) == 1 {
        permutations = [][]int{elements}
        return permutations
    }
    for i := range elements {
        el := make([]int, len(elements))
        copy(el, elements)

        // or copy via append
        // el := append([]int(nil), elements...)

        for _, perm := range getPermutations(append(el[0:i], el[i+1:]...)) {
            permutations = append(permutations, append([]int{elements[i]}, perm...))
        }
    }
    return permutations
}

https://play.golang.org/p/oewV8iPd8E