[template record] string hash to judge palindrome string

Zhejiang Province competition The first I topic Barbecue

The third place of Zhejiang provincial competition I topic Barbecue, Use string hash to do .

Hash the palindrome string ： Calculate the past hash value of a string , Then calculate the hash value of the string in reverse . Then compare the hash value of positive and negative strings , Equality means palindrome .

Reference blog ：I. Barbecue_giao Source's blog -CSDN Blog

character string hash Algorithm solves palindrome string - SegmentFault Think no

Post the following code , When you encounter the same problem in the future, use it as a template .（ In fact, I wrote several hash methods and comparisons , But this kind of relatively speaking is best understood ）

#include <iostream>
#include <string>
using namespace std;
typedef unsigned long long ull;
#define base 2333
const ull  N = 1e6 + 5;
ull hash1[N];
ull hash2[N];
ull prev_t[N];
int main()
{
    
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	int n, q;
	cin >> n >> q;
	string str;
	cin >> str;
	int len = str.size();
	prev_t[0] = 1;
	hash1[0] = 0;
	hash2[0] = 0;
	for (int i = 1; i <= len; i++)// The string subscript is i Corresponding to hash array i+1
	{
    
		hash1[i] = hash1[i - 1] * base + str[i - 1] - 'a' + 1;
		hash2[i] = hash2[i - 1] * base + str[len - i] - 'a' + 1;// The hash value of the reverse string 
		prev_t[i] = prev_t[i - 1] * base;// The array works when getting the hash value of the substring 
	}
	while (q--)
	{
    
		int l, r;
		cin >> l >> r;
		int str_lr = r - l + 1;
		if (str_lr % 2 == 0)
		{
    
			cout << "Budada\n" ;
		}
		else if ((hash1[r] - hash1[l - 1] * prev_t[r - l + 1]) ==
			hash2[len - l + 1] - hash2[len - r] * prev_t[r - l + 1])// Compare the 
		{
    
			cout << "Budada\n";
		}
		else
		{
    
			cout << "Putata\n";
		}
	}
	return 0;
}