1354. Palindrome. Again Palindrome

gistfile1.cpp

#include <vector>
#include <string>
#include <iostream>
#include <algorithm>

void z_function (const std::string & s, std::vector<int> & z) 
{
	size_t length = s.length();
	z.clear();
	z.resize(length);
	for (int i = 1, l = 0, r = 0; i < length; ++i) 
	{
		if (i <= r)
			z[i] = std::min (r-i+1, z[i-l]);
		while (i+z[i] < length && s[z[i]] == s[i+z[i]])
			++z[i];
		if (i+z[i]-1 > r)
			l = i,  r = i+z[i]-1;
	}
}

size_t find_suff_length(const std::vector<int> & z)
{
	size_t string_length = (z.size() - 1) / 2;
	for(auto it = z.begin() + string_length + 2; it != z.end(); ++it)
	{
		if (*it == std::distance(it, z.end()))
			return *it;
	}
	return 0;
}

int main()
{
	//freopen("C:\\temp\\input.txt", "r", stdin);
	std::vector<int> pi;
	std::string st;
	std::cin >> st;
	std::string reversed_st = st;
	std::reverse(reversed_st.begin(), reversed_st.end());
	std::string final_string = reversed_st + "#" + st;
	std::vector<int> z;
	z_function(final_string, z);
	auto max = find_suff_length(z);
	std::string answer = st + reversed_st.substr(max, st.length() - max);
	std::cout << answer << std::endl;
	return 0;
}