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] = max(dp[i+1][j], max(dp[i][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 <iostream>
#include <algorithm>
#include <string>
#include <fstream>

using namespace std;

int check(string, string);

int main()
{
  //ifstream ifs("data.txt");
  string str1, str2;
  while( cin >> str1 >> str2 ){

    int res = 0;

    for(int i = 0; i < str1.size(); i++){
      rotate(str1.begin(), str1.begin() + 1, str1.end());

      res = max(res, check(str1, str2));
    }

    for(int i = 0; i < str2.size(); i++){
	rotate(str2.begin(), str2.begin() + 1, str2.end());

      res = max(res, check(str1, str2));
    }

    cout << res << endl;
  }
  //ifs.close();
}

inline int check(string str1, string str2)
{
  int sn = 0, res = 0;
  for(int i = 0; i < min(str1.size(), str2.size()); i++){
    if(str1[i] == str2[i]){
      sn++;
    }else{
      sn = 0;
    }
    res = max(res, sn);
  }
  return res;
}