func summaryRanges(nums []int) (ans []string) {
    for i, n := 0, len(nums); i < n; {
        left := i
        for i++; i < n && nums[i-1]+1 == nums[i]; i++ {
        }
        s := strconv.Itoa(nums[left])
        if left < i-1 {
            s += "->" + strconv.Itoa(nums[i-1])
        }
        ans = append(ans, s)
    }
    return
}