46. Permutations.go

回溯算法
需要注意的是，填充结果的时候，需要copy一个slice

func permute(nums []int) [][]int {
    var ret [][]int
    l := len(nums)
    if l == 0 {
        return ret
    }
    helper(nums, 0, l-1, &ret)
    return ret
}

func helper(nums []int, begin, end int, ret *[][]int) {
    if begin == end {
        t := make([]int, len(nums))
        copy(t, nums) //这里一定要copy
        *ret = append(*ret, t)
        return
    }

    for i := begin; i <= end; i++ {
        nums[begin], nums[i] = nums[i], nums[begin]
        helper(nums, begin+1, end, ret)
        nums[begin], nums[i] = nums[i], nums[begin]
    }
}