《字符串专题》

查询回文子串个数：

#include<bits/stdc++.h>
using namespace std;
typedef long long LL;
typedef pair<double,int> pii;
const int N = 3e5+5;
const int M = 1e6+5;
const LL Mod = 1e9+7;
#define rg register
#define pi acos(-1)
#define INF 1e18
#define CT0 cin.tie(0),cout.tie(0)
#define IO ios::sync_with_stdio(false)
#define dbg(ax) cout << "now this num is " << ax << endl;
namespace FASTIO{
    inline LL read(){
        LL x = 0,f = 1;char c = getchar();
        while(c < '0' || c > '9'){if(c == '-') f = -1;c = getchar();}
        while(c >= '0' && c <= '9'){x = (x<<1)+(x<<3)+(c^48);c = getchar();}
        return x*f;
    }
    void print(int x){
        if(x < 0){x = -x;putchar('-');}
        if(x > 9) print(x/10);
        putchar(x%10+'0');
    }
}
using namespace FASTIO;

int len[N];
int Marchar(string t)
{
    string s = "@#";
    int mx = 0,cnt = 0,p = 0;
    for(rg int i = 0;i < t.size();++i)
    {
        s += "#";
        s += t[i];
    }
    s += "#";
    for(rg int i = 0;i < s.size();++i)
    {
        if(mx > i) len[i] = min(mx-i,len[2*p-i]);
        else len[i] = 1;
        while(s[i-len[i]] == s[i+len[i]]) len[i]++;
        if(len[i]+i > mx) mx = len[i]+i,p = i;
        cnt += len[i]/2;
    }
    return cnt;
}
int main()
{
    int n;n = read();
    string s;cin >> s;
    int ans = Marchar(s);
    printf("%d\n",ans);
    system("pause");
    return 0;
}