#include <iostream>
using namespace std;

int main() {
	std::string str{ "[1,2,3,,,,,4]" };
	do {
		auto pos = str.find( ",," );
		if( pos == std::string::npos )
		    break;
		str.insert( pos + 1, "null" );
	} while( true );
	std::cout << str << std::endl;
	return 0;
}