int 转 string：
#include <sstream>
string int_to_string(const int i){
stringstream stream;
stream<<i;
string s;
stream >> s;
return s;
}string 转 int：
#include <sstream>
int string_to_int(const string s){
stringstream stream;
stream<<s;
int i;
stream >> i;
return i;
}