https://www.codewars.com/kata/51ba717bb08c1cd60f00002f/train/python
​连续整数的缩写

表达整数有序列表的格式是使用逗号分隔的列表，其内容包括一个整数范围，用破折号'-'隔开起始整数和结束整数。范围包括区间内的所有整数，包括两个端点。除非它跨越至少3个数字，否则不被认为是一个范围。例如（"12，13，15-17"）。

完成解题，使其以递增顺序接收一个整数列表，并以范围格式返回一个正确格式化的字符串。
​输入：
[-6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20]
​
输出："-6,-3-1,3-5,7-11,14,15,17-20"

任务选自codewars题库难度 4kyu：
A format for expressing an ordered list of integers is to use a comma separated list of either

individual integers
or a range of integers denoted by the starting integer separated from the end integer in the range by a dash, '-'. The range includes all integers in the interval including both endpoints. It is not considered a range unless it spans at least 3 numbers. For example ("12, 13, 15-17")
Complete the solution so that it takes a list of integers in increasing order and returns a correctly formatted string in the range format.

Example:

solution([-6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20])

returns "-6,-3-1,3-5,7-11,14,15,17-20"

Test.describe("Sample Test Cases")

Test.it("Simple Tests")
Test.assert_equals(solution([-6,-3,-2,-1,0,1,3,4,5,7,8,9,10,11,14,15,17,18,19,20]), '-6,-3-1,3-5,7-11,14,15,17-20')
Test.assert_equals(solution([-3,-2,-1,2,10,15,16,18,19,20]), '-3--1,2,10,15,16,18-20')

def solution(args):
    out = []
    beg = end = args[0]

    for n in args[1:] + [""]:
        if n != end + 1:
            if end == beg:
                out.append(str(beg))
            elif end == beg + 1:
                out.extend([str(beg), str(end)])
            else:
                out.append(str(beg) + "-" + str(end))
            beg = n
        end = n

    return ",".join(out)