归并排序

func mergeSort(r []int) []int {
    length := len(r)   
    if length <= 1 {
        return r 
    }   
    num := length / 2
    left := mergeSort(r[:num])   
    right := mergeSort(r[num:])   
    return merge(left, right)
}
func merge(left, right []int) (result []int) {   
    l, r := 0, 0   
    for l < len(left) && r < len(right) {
        if left[l] < right[r] {         
            result = append(result, left[l])         
            l++      
        } else {         
            result = append(result, right[r])         
            r++      
        }   
    }   
    result = append(result, left[l:]...)   
    result = append(result, right[r:]...)   
    return
}