void erase( iterator pos );
void erase( iterator start, iterator end );
size_type erase( const KEY_TYPE &key );
map<int,int> mp;
mp[1] = 2;
mp[2] = 3;
mp[3] = 4;
map<int,int>::size_type ret = mp.erase(0);
printf("retis %u\n",ret);//打印0
ret = mp.erase(3);
printf("ret is %u\n",ret);//打印1