This problem will help you understand how to use std::map.
http://en.cppreference.com/w/cpp/container/map
Notice, this time, first and last is [first, last].
目前有五個測資，第三筆測資不算，答題測資看另外四筆
The input consist of series of command. Each commands is insert, range output or range erase.
insert: inserts a string (cmd) with the key (key) to map. If the key has already existed, insert the cmd at the begining of string (the string which the key belongs).
For example,
insert 0 "abc"
insert 1 "def"
the map should contain "abc", "def".
insert 0 "xyz"
the map should contain "xyzabc", "def".
range output: outputs the string from key (first) to key (last). Output a space character (' ') after printing an element.
range erase: erase the string from key (first) to key (last).
operator<<: outputs all strings in map. From the smallest key to biggest key. Output a space character (' ') after printing an element.
Complete insert, output, erase and operator<<.