substrWith2DiffChars

// only for one-byte encoding

string get2CharSubString(const string& s) {
	auto l = s.length();
	if (l < 3) return s;

	size_t p = 0; // start max substr position
	size_t m = 0; // current max length of substr
	char c1 = s[0];
	char c2 = '\0';

	for (size_t i = 1; i < l; i++) {
		// cout << "new step, start pos=" << i << " for char=" << c1 << endl;
		if (s[i] != c1) {
			c2 = s[i];
			// we have two different chars and start index for search
			// search to the left
			size_t cntLeft = 0;
			for (int j = i - 1; j >= 0; j--) {
				if (s[j] == c1 || s[j] == c2) {
					cntLeft++;
					continue;
				}
				break;
			}
			// search to the right 
			size_t cntRight = 0;
			for (size_t j = i + 1; j < l; j++) {
				if (s[j] == c1 || s[j] == c2) {
					cntRight++;
					continue;
				}
				break;
			}

			// update max 
			if (m < cntLeft + cntRight + 1) {
				p = i - cntLeft;
				m = cntLeft + cntRight + 1; // 1 = current char
			}

			//cout << "p=" << p << " len=" << m << "[" << cntLeft << "," << cntRight << "] [" << c1 << "," << c2 << "]\n";

			// update vars
			i += cntRight;
			c1 = c2;
		}
	}
	return string(s.c_str() + p, m);
}