string:substr/3
截取字符串
用法：
substr(String, Start, Length) -> Substring
从字符串 String 里截取一个从 Start 位置开始，长度是 Length 的子字符串 Substring。
string:substr("abcdefg", 3, 2).
Erlang 版实现的全排列：
Str = "abc",
Len = length(Str),
StrToListFun = fun(Start, AccList) ->
Element = string:substr(Str, Start, 1),
[Element | AccList]
end,
StringList = lists:foldr(StrToListFun, [], lists:seq(1, Len)),
NormalizeFun = fun
Normalize([], _N, _I, R) ->
lists:reverse(R);
Normalize([H | L], I, I, R) ->
[H] ++ lists:reverse(R) ++ L;
Normalize([H | L], N, I, R) ->
Normalize(L, N + 1, I, [H | R])
end,
PermutationFun = fun PermutationFun(N, [StrList, HeadList, Ret]) ->
NewStrList = NormalizeFun(StrList, 1, N, []),
IOStrList = HeadList ++ NewStrList,
NewRet = [string:join(IOStrList, "") | Ret],
if
length(NewStrList) > 1 ->
[Item | AccStrList] = NewStrList,
AccLen = length(AccStrList),
NewHeadList = HeadList ++ [Item],
[_SL, _HL, PermutationLoopRet] = lists:foldr(PermutationFun, [AccStrList, NewHeadList, []], lists:seq(2, AccLen)),
[StrList, HeadList, PermutationLoopRet ++ NewRet];
true ->
[StrList, HeadList, NewRet]
end
end,
[_StrList, _HeadList, RetList] = lists:foldr(PermutationFun, [StringList, [], []], lists:seq(1, Len)),
RetList.