наибольшая подстрока

#include <fstream>
#include <string>
#include <map>

using namespace std;

const int MAXLEN = 100000;

int sz, last;

struct state {
    int len, link;
    map<char, int> next;
};

state st[MAXLEN * 2];

void init() {
    sz = last = 0;
    st[0].len = 0;
    st[0].link = -1;
    ++sz;
}

void extend(char c) {
    int cur = sz++;
    st[cur].len = st[last].len + 1;
    int p;
    for (p = last; p != -1 && !st[p].next.count(c); p = st[p].link)
        st[p].next[c] = cur;
    if (p == -1)
        st[cur].link = 0;
    else {
        int q = st[p].next[c];
        if (st[p].len + 1 == st[q].len)
            st[cur].link = q;
        else {
            int clone = sz++;
            st[clone].len = st[p].len + 1;
            st[clone].next = st[q].next;
            st[clone].link = st[q].link;
            for (; p != -1 && st[p].next[c] == q; p = st[p].link)
                st[p].next[c] = clone;
            st[q].link = st[cur].link = clone;
        }
    }
    last = cur;
}


string lcs(string s, string t) {
    init();
    for (int i = 0; i < (int) s.length(); ++i)
        extend(s[i]);

    int v = 0, l = 0,
            best = 0, bestpos = 0;
    for (int i = 0; i < (int) t.length(); ++i) {
        while (v && !st[v].next.count(t[i])) {
            v = st[v].link;
            l = st[v].len;
        }
        if (st[v].next.count(t[i])) {
            v = st[v].next[t[i]];
            ++l;
        }
        if (l > best)
            best = l, bestpos = i;
    }
    return t.substr(bestpos - best + 1, best);
}

int main(){
    ifstream fin("common.in");
    ofstream fout("common.out");

    string first, second;
    fin >> first >> second;
    fout << lcs(first, second);
}