bzoj 2342: [Shoi2011]双倍回文

2333，做这个题的时候IQ--，不说了，GG

#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <set>
#define LL long long
using namespace std;

const int maxn=500000+10;

int n,ans;
char ch[maxn];
int p[maxn],q[maxn];
set<int> t;

void manachar()
{
    int mx=0,id=0;
    for (int i=1; i<=n; i++)
    {
        if (mx>=i) p[i]=min(p[2*id-i],mx-i); else p[i]=0;
        while (ch[i+p[i]+1]==ch[i-p[i]]) p[i]++;
        if (p[i]+i>mx) id=i,mx=p[i]+i;
    }
}

bool cmp(int a, int b){return a-p[a]<b-p[b];}

int main()
{
    scanf("%d%s",&n,ch+1); ch[0]='#'; manachar();
    for (int i=1; i<=n; i++) q[i]=i;
    sort(q+1,q+n+1,cmp);
    int now=1;
    for (int i=1; i<=n; i++)
    {
        while (now<=n && q[now]-p[q[now]]<=i) t.insert(q[now++]);
        set<int>::iterator tmp=t.upper_bound(i+p[i]/2);
        if (tmp!=t.begin()) ans=max(ans,(*--tmp-i)*4); 
    }
    cout<<ans<<endl;
    return 0;
}