multimap的元素添加和删除:
multimap<string,string> author;
string search_item("kaozuo Ishi");
//erase all elements with this key;return number of elements removed
multimap<string,string>::size_type cnt=author.erase(search_item); //返回删除元素的个数
author.insert(make_pair(string("Barth,John"),string(Sot_Weed))); //添加元素