According to this, we can use to_string() only with compiler higher than GCC 4.8.0 and with C++ 11.
But I submitted with GNU C++ 11 4.9.2 (12720137) and got this error.
I know that are other solutions to convert int to string, but I want to know what is problem with this one. I am using GNU C++ 11 5.1.0 and it's working on my computer. Thanks.