用库的话更简单，想自己写的话用递归也直观， 只需要想清楚C(m,n) = C(m-1,n) + C(m-1,n-1)，就可以写递归函数了
python:
def all_combine(str):
    res = []
    for i in range(len(str)):
        res = res + combine(str,i+1)
    return res
        
# 返回s字符长度为n 的组和，s里没有重复字符
def combine(s, n):
    if n < 1:
        return []
    if n == 1:
        return [c for c in s]
    if len(s) < 1:
        return []

    return [s[0] + p for p in combine(s[1:], n-1)] + combine(s[1:], n)
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FROM 123.126.24.*