spoj Longest Common Substring

Longest Common Substring

SPOJ - LCS

题意：求两个串的最长公共子串

/*
    对一个串建立后缀自动机，用另一个串上去匹配
*/
#include<iostream>
#include<cstdio>
#include<cstring>
#define maxn 250002
using namespace std;
char s1[maxn],s2[maxn];
int ch[maxn<<][],fa[maxn<<],tot=,len[maxn<<],last=,p,np,q,nq;
void Insert(int c){
    np=++tot;
    len[np]=len[last]+;
    p=last;
    while(p&&!ch[p][c])ch[p][c]=np,p=fa[p];
    if(!p)fa[np]=;
    else {
        q=ch[p][c];
        if(len[q]==len[p]+)fa[np]=q;
        else {
            nq=++tot;
            memcpy(ch[nq],ch[q],sizeof(ch[nq]));
            fa[nq]=fa[q];
            fa[q]=fa[np]=nq;
            len[nq]=len[p]+;
            while(ch[p][c]==q)ch[p][c]=nq,p=fa[p];
        }
    }
    last=np;
}
void solve(){
    int l=strlen(s2+),now=,ans=,now_len=,c;
    for(int i=;i<=l;i++){
        c=s2[i]-'a';
        while(now&&!ch[now][c]){
            now=fa[now];
            now_len=len[now];
        }
        if(!now){
            now_len=;
            now=;
        }
        else if(ch[now][c]){
            now_len++;
            now=ch[now][c];
            ans=max(ans,now_len);
        }
    }
    printf("%d",ans);
}
int main(){
    scanf("%s%s",s1+,s2+);
    int l=strlen(s1+);
    for(int i=;i<=l;i++)Insert(s1[i]-'a');
    solve();
    return ;
}