哈希表

#include<cstdio>
#include<iostream>
#include<queue>
#include<algorithm>
#define P1 20031101
#define P2 998244353
#define N 1000000
#define int long long
using namespace std;
int head[30000000],nxt[N],to[N];
int l[N],h1[N],h2[N],pw1[N],pw2[N];
char s[N];
int n,cnt,p=97;
int read()
{
    int x=0,f=1;
    char ch=getchar();
    while(ch<'0'||ch>'9')
    {
    if(ch=='-') f=-1;
    ch=getchar();
    }
    while(ch>='0'&&ch<='9') x=(x<<3)+(x<<1)+ch-'0',ch=getchar();
    return x*f;
}
void init()
{
    pw1[0]=pw2[0]=1;
    for(int i=1; i<=n; i++)
    {
    h1[i]=(h1[i-1]*p+s[i])%P1;
    pw1[i]=pw1[i-1]*p%P1;
    h2[i]=(h2[i-1]*p+s[i])%P2;
    pw2[i]=pw2[i-1]*p%P2;
    }
}
void clear()
{
    for(int i=1; i<=cnt; i++) head[l[i]]=0;
    cnt=0;
}
bool find(int x,int y)
{
    for(int i=head[x]; i; i=nxt[i]) if(to[i]==y) return 1;
    l[++cnt]=x;
    to[cnt]=y;
    nxt[cnt]=head[x];
    head[x]=cnt;
    return 0;
}
bool check(int x)
{
    if(x==0) return 1;
    clear();
    for(int i=1; i<=n-x+1; i++)
    if(find((P1+h1[i+x-1]-h1[i-1]*pw1[x]%P1)%P1,(P2+h2[i+x-1]-h2[i-1]*pw2[x])%P2)) return 1;
    return 0;
}
void solve()
{
    int L=0,R=n;
    int ans=-1;
    while(L<=R)
    {
    int mid=(L+R)>>1;
    if(check(mid)) ans=mid,L=mid+1;
    else R=mid-1;
    }
    printf("%lld\n",ans);
}
main()
{
    n=read();
    scanf("%s",s+1);
    init();
    solve();
    return 0;
}