Go: ///
// File:  0528.go
// Author: ymiyamoto
//
// Created on Thu Mar 22 01:52:09 2018
//
package main

import (
	"bufio"
	"fmt"
	"os"
)

var S, T string
var dp [4001][4001]int

func max(a, b int) int {
	if a < b {
		return b
	}
	return a
}

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	for {
		scanner.Scan()
		S = scanner.Text()
		if len(S) == 0 {
			return
		}

		scanner.Scan()
		T = scanner.Text()

		for i := range dp {
			for j := range dp[i] {
				dp[i][j] = 0
			}
		}

		dp[0][0] = 0
		for i := range S {
			for j := range T {
				if S[i] == T[j] {
					dp[i+1][j+1] = dp[i][j] + 1
				} else {
					dp[i+1][j+1] = 0
				}
			}
		}

		var m int
		for i := range dp {
			for j := range dp[i] {
				m = max(m, dp[i][j])
			}
		}
		fmt.Println(m)
	}
}

C++: #include <bits/stdc++.h>

using namespace std;

struct RollingHash {
  typedef unsigned long long hash_t;
  static const hash_t P = 1469979569;
  static const hash_t M = 1ll << 63;
  const int n;
  vector<hash_t> pow, phash;
  RollingHash(const string& s) : n(s.size()), pow(n + 1), phash(n + 1) {
    pow[0] = 1;
    phash[0] = 0;
    for (int i = 0; i < n; i++){
      phash[i + 1] = (s[i] + phash[i] * P) % M;
      pow[i + 1] = (pow[i] * P) % M;
    }
  }
  hash_t hash(int i, int j){
    return (phash[j] - phash[i] * pow[j - i]) % M;
  }
  int lcp(int i, int j){
    int l = 0, r = min(n - i, n - j) + 1;
    while (r - l > 1){
      int m = r + l >> 1;
      if (hash(i, i + m) == hash(j, j + m)) l = m;
      else r = m;
    }
    return l;
  }
};

typedef long long ll;
typedef long double ld;
typedef tuple<int, int> duo;
const int dx[] = {0, 0, 1, -1, 1, 1, -1, -1};
const int dy[] = {1, -1, 0, 0, 1, -1, 1, -1};
const int Mod = 1000000000 + 0;
//{{{ templates
#define TT_ template<typename T>inline
#define TTF_ template<typename T,typename F>inline
TT_ T sq(T x){return x*x;}
TT_ T In(){T x;cin>>x;return x;}
TT_ void Out(T&x){cout<<x;}
TT_ void sort(T&v){sort(begin(v),end(v));}
TT_ void revs(T&v){reverse(begin(v),end(v));}
TT_ void uniq(T&v){sort(v);v.erase(unique(begin(v),end(v)),end(v));}
TT_ int ubnd(T&v,typename T::value_type&x){return upper_bound(begin(v),end(v),x)-begin(v);}
TT_ int lbnd(T&v,typename T::value_type&x){return lower_bound(begin(v),end(v),x)-begin(v);}
TTF_ void inpt(T&v,int n,F f){for(v.reserve(n);n--;v.emplace_back(f()));}
TTF_ void show(T&v,F f,string d=" ",string e="\n"){int i=0;for(auto&x:v)i++&&(cout<<d),f(x);cout<<e;}
TT_ typename T::iterator minel(T&v){return min_element(begin(v),end(v));}
TT_ typename T::iterator maxel(T&v){return max_element(begin(v),end(v));}

inline void fast_io(){ios::sync_with_stdio(0);cin.tie(0);}
inline int in(){int x;scanf("%d",&x);return x;}
inline ll pow_mod(ll a,ll k,ll m){ll r=1;for(;k>0;a=a*a%m,k>>=1)if(k&1)r=r*a%m;return r;}
inline ll mod_inv(ll a,ll p){return pow_mod(a,p-2,p);}
//}}} priority_queue queue deque front stringstream max_element min_element insert count make_tuple

int main()
{
  string s, t;
  while (cin >> s >> t){
    string S = s + '$' + t;
    RollingHash rh(S);
    int maxi = 0;
    for (int i = 0; i < s.size(); i++){
      for (int j = 0; j < t.size(); j++){
        maxi = max(maxi, rh.lcp(i, s.size() + 1 + j));
      }
    }
    cout << maxi << endl;
  }
  return 0;
}

C++: #include <bits/stdc++.h>
#define r(i,n) for(int i=0;i<n;i++)
using namespace std;
string s,t;
set<string>st;
int main(){
	cin>>s>>t;
	r(j,s.size())if(j)r(i,s.size())if(j+i<=s.size())
	  st.insert(s.substr(i,j));
	for(int j=t.size()-1;j>=1;j--)if(j)r(i,t.size())if(j+i<=t.size())
		if(st.count(t.substr(i,j))){
			cout<<j<<endl;
			return 0;
		}
	cout<<0<<endl;
}

C++: #include <bits/stdc++.h>
using namespace std;
// #define int long long   // <-----!!!!!!!!!!!!!!!!!!!

#define rep(i,n) for (int i=0;i<(n);i++)
#define rep2(i,a,b) for (int i=(a);i<(b);i++)
#define rrep(i,n) for (int i=(n)-1;i>=0;i--)
#define rrep2(i,a,b) for (int i=(a)-1;i>=b;i--)
#define chmin(a,b) (a)=min((a),(b));
#define chmax(a,b) (a)=max((a),(b));
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()
#define printV(v) cout<<(#v)<<":";for(auto(x):(v)){cout<<" "<<(x);}cout<<endl;
#define printVS(vs) cout<<(#vs)<<":"<<endl;for(auto(s):(vs)){cout<<(s)<< endl;}
#define printVV(vv) cout<<(#vv)<<":"<<endl;for(auto(v):(vv)){for(auto(x):(v)){cout<<" "<<(x);}cout<<endl;}
#define printP(p) cout<<(#p)<<(p).first<<" "<<(p).second<<endl;
#define printVP(vp) cout<<(#vp)<<":"<<endl;for(auto(p):(vp)){cout<<(p).first<<" "<<(p).second<<endl;}

inline void output(){ cout << endl; }
template<typename First, typename... Rest>
inline void output(const First& first, const Rest&... rest) {
    cout << first << " "; output(rest...);
}

using ll = long long;
using Pii = pair<int, int>;
using TUPLE = tuple<int, int, int>;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
const int inf = 1e9;
const int mod = 1e9 + 7;
using Graph = vector<vector<int>>;

string s, t;
int n, m;
int dp[4001][4001];

signed main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);

    while (cin >> s >> t) {
        n = s.size(); m = t.size();
        int ans = 0;
        rep(i, n) {
            rep(j, m) {
                if (i == 0 || j == 0) dp[i][j] = 0;
                if (s[i] == t[j]) {
                    dp[i + 1][j + 1] = dp[i][j] + 1;
                    chmax(ans, dp[i + 1][j + 1]);
                }
                else {
                    dp[i + 1][j + 1] = 0;
                }
            }
        }
        cout << ans << endl;
    }
}

C++: #include <iostream>
#include <iomanip>
#include <vector>
#include <set>
#include <map>
#include <list>
#include <iterator>
#include <queue>
#include <stack>
#include <deque>
#include <sstream>
#include <algorithm>
#include <numeric>
#include <string>
#include <cmath>
#include <complex>
#include <functional>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cassert>
#include <cstdlib>
#include <cctype>

#define REP(i,m,n) for(int i=m;i<(int)n;i++)
#define rep(i,n) for(int i=0;i<(int)n;i++)
#define rrep(i,n) for(int i = 1; i <= n; i++)
#define drep(i,n) for(int i = n; i >= 0; i--)
#define rp(i,c) rep(i,(int)c.size())
#define fr(i,c) for(__typeof((c).begin()) i=(c).begin();i!=(c).end();i++)
#define mp make_pair
#define pb push_back
#define all(c) (c).begin(),(c).end()
#define rall(u) (u).rbegin(),(u).rend()
#define dbg(x) cerr<<#x<<" = "<<(x)<<endl
#define v_delete(a,b) (a).erase(remove((a).begin(), (a).end(), b), (a).end())
#define v_unique(a) (a).erase(unique((a).begin(), (a).end()), (a).end())
#define VV(T) vector<vector< T > >
#define init(a,b) memset((a), (b), sizeof((a)))
using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> pii;
const int inf=1<<28;
const double INF=1e10,EPS=1e-9;


template<class T>int SIZE(T a){return a.size();}
template<class T>string IntToString(T num){string res;stringstream ss;ss<<num;return ss.str();}
template<class T>T StringToInt(string str){T res=0;for(int i=0;i<SIZE(str);i++)res=(res*10+str[i]-'0');return res;}
template<class T>T gcd(T a,T b){if(b==0)return a;return gcd(b,a%b);}
template<class T>T lcm(T a,T b){return a/gcd(a,b)*b;}

template <class T> void input(T& a, int n) { for (int i = 0; i < n; ++i) cin >> a[i]; }
template <class T> void input(T* a, int n) { for (int i = 0; i < n; ++i) cin >> a[i]; }

int dp[4002][4002];
int main(){
	string a,b;
	while(cin >> a >> b){
        for(int i=0;i<4001;i++)dp[i][0]=dp[0][i]=0;
	int as=a.size();
	int bs=b.size();
	int ans=0;
	for(int i=1;i<=as;i++){
		for(int j=1;j<=bs;j++){
			if(a[i-1]==b[j-1]){
				dp[i][j]=dp[i-1][j-1]+1;
			}else{
				dp[i][j]=0;
			}
			ans=max(ans,dp[i][j]);

		}
	}
	cout << ans << endl;
	}
	return 0;
}