#include <iostream>
#include <locale>

#include <string>
#include <set>
#include <functional>

bool good(const std::string& s)
{
	std::set<char> unique;
	for (std::string::const_iterator it = s.begin(); it != s.end(); ++it)
	{
		unique.insert(*it);
		if (unique.size() > 2)
			return false;
	}

	return true;
}

template <typename T>
struct size_less : public std::binary_function<T, T, bool>
{
	bool operator()(const T& v1, const T& v2) const
	{ return v1.size() < v2.size(); }
};

std::string find(const std::string& s)
{
	if (s.size() == 1)
		return s;

	std::set<std::string, size_less<std::string> > v;

	for (size_t i=0; i<s.size(); i++)
	for (size_t j=i+1; j<s.size(); j++)
	{
		const std::string sub = s.substr(i, j-i+1);
		if (good(sub))
			v.insert(sub);
	}

	return *v.rbegin();
}

int main()
{
	setlocale(LC_ALL, "");

	const std::string s = "arrdeddsrdsreewdrdcegt";
	std::cout << s << " => " << find(s) << std::endl;

	return 0;
}

arrdeddsrdsreewdrdcegt => dedd