erlang 题目：一个integer列表，按照数字出现的次数由少到多排序，相同的数字小 的在前面

比如 列表[1,1,1,1,2,2,2,4,4,4,4,4,4,3,3,5,6,6,6] 处理成
[5,3,3,2,2,2,6,6,6,1,1,1,1,4,4,4,4,4,4]

% -export([sort/1]).
-compile(export_all).
get_map(List) ->
    lists:foldl(
    	fun(X, Map1) ->
    	    % io:format("new map is ~p~n", [Map1]),
    	    case maps:find(X, Map1)  of
    		    error ->
    		        % io:format("before add map is ~p~n", [Map1]),
    		        maps:put(X, 1, Map1);
    		    {ok, Value} ->
    		        % io:format("before update map is ~p~n", [Map1]),
    		        maps:update(X, Value + 1, Map1)
    		end 
        end, #{}, List).

sort(List) ->
    Map = get_map(List),
    List1 = maps:to_list(Map),
    List2 = lists:keysort(2, List1),
    List4 = lists:foldl(
        fun({Key, Value}, List3) ->
            [lists:duplicate(Value, Key) | List3]
        end, [], List2),
    lists:flatten(lists:reverse(List4)).

test_sort() ->
    List = [1,1,1,1,2,2,2,4,4,4,4,4,4,3,3,5,6,6,6],
    sort(List).

测试：
1> c(sort_by_times).
{ok,sort_by_times}
2> sort_by_times:test_sort().
[5,3,3,2,2,2,6,6,6,1,1,1,1,4,4,4,4,4,4]
3> sort_by_times:get_map([1,1,3,3,3,3,2,2,2,2,2,6,6,6]).
#{1 => 2,2 => 5,3 => 4,6 => 3}
4> Map =sort_by_times:get_map([1,1,3,3,3,3,2,2,2,2,2,6,6,6]).
#{1 => 2,2 => 5,3 => 4,6 => 3}
5>