[luogu] SP7586 NUMOFPAL - Number of Palindromes 字符串hash

前言

字符串hash也太难了

思路

前置: https://www.bilibili.com/video/BV12T4y1g7Vc?from=search&seid=12446137755286056736&spm_id_from=333.337.0.0

BKRD-hash方法 进行hash操作

然后反着操作一下

最后二分比较hash数组

CODE

#include 
using namespace std;
using ll =unsigned long long;
const int N = 1009;
ll mi[N],h1[N],h2[N],ans;
const int x =13;
int n ;
string s;
int len ;

ll gethash1(int x,int y)
{
    return h1[y] - h1[x-1] * mi[y-x+1];
}

ll gethash2(int x,int y)
{
    return h2[x] - h2[y+1] * mi[y-x+1];
}
ll query1(int x)
{
    int  l =1 , r=min(x,len-1-x);
    while(l>1;
        if(gethash1(x-mid,x+mid)==gethash2(x-mid,x+mid))
            l=mid+1;
        else
            r=mid-1;
    }
    return r;
}
ll query2(int x)
{
 int  l =1 , r=min(x,len-1-x);
    while(l>1;
        if(gethash2(x-mid+1,x+mid)==gethash1(x-mid+1,x+mid))
            l=mid+1;
        else
            r=mid-1;
    }
    return r;
}
void solve()
{
    string t ;
    cin>>t;
    s = " ";s+=t;
    len =s.size();



    mi[0] = 1;

    for(int i=1;i=1;i -- )
    {
        h2[i] = h2[i+1]*x+(s[i]-'a');
    }


    for(int i = 1;i