bzoj 1318: [Spoj744] Longest Permutation 智商题

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cstdio>
using namespace std;
#define MAXN 101000
typedef long long qword;
int a[MAXN];
int prv[MAXN];
int pid[MAXN];
int vis[MAXN];
int l[MAXN];
qword s[MAXN];
int bstans=0;
void solve(int n)
{
        memset(pid,0,sizeof(pid));
        for (int i=1;i<=n;i++)
        {
                if (a[i]>n)
                {
                        prv[i]=0;
                        a[i]=n+1;
                }
                else
                {
                        prv[i]=pid[a[i]];
                        pid[a[i]]=i;
                }
        }
        for (int i=1;i<=n;i++)
                s[i]=s[i-1]+a[i];
        for (int i=1;i<=n;i++)
        {
                if (a[i]==1)
                {
                        int mx=0;
                        for (int j=i+1;j<=n && a[j]!=1;j++)
                                l[j]=min(l[j-1],prv[j]);
                        int prvmax=0;
                        for (int j=i;j>=1 && (j==i || a[j]!=1);j--)
                        {
                                mx=max(mx,a[j]);
                                prvmax=max(prvmax,prv[j]);
                                if (j+mx-1<i || j+mx-1>n)continue;
                                if (max(prvmax,l[j+mx-1])>=j)continue;
                                if (s[j+mx-1]-s[j-1]==(qword)(mx+1)*mx/2)
                                {
                                        bstans=max(bstans,mx);
                                }
                        }
                }
        }
}

int main()
{
        freopen("input.txt","r",stdin);
        int n;
        scanf("%d",&n);
        int l,r;
        for (int i=1;i<=n;i++)
                scanf("%d",a+i);
        solve(n);
        for (int i=1;i<=n/2;i++)
                swap(a[i],a[n-i+1]);
        solve(n);
        printf("%d\n",bstans);
}