Given two input texts, find the longest string that occurs in both.

longest.cpp

#include <iostream>
#include <string>
#include <vector>
#include <math.h>
using namespace std;

string longest_in_both(const string& s1, const string& s2) {
    vector<vector<int> > dp;
    for(int i = 0; i < s1.length(); i++) {
        dp.push_back(vector<int>(s2.length(), -1));
    }

    for(int i = 0; i < s1.length(); i++) {
        dp[0][i] = (s1[0] == s2[i])? 1:0;
        dp[i][0] = (s1[i] == s1[0])? 1:0;
    }

    for(int i = 1; i < s1.length(); i++) {
        for(int j = 1; j < s2.length(); j++) {
            dp[i][j] = (s1[i] == s2[j])? 1+dp[i-1][j-1]:0;
        }
    }

    int m = -1;
    int mark1 = -1;
    int mark2 = -1;
    for(int i = 0; i < s1.length(); i++) {
        for(int j = 0; j < s2.length(); j++) {
            if(dp[i][j] > m) {
                mark1 = i;
                mark2 = j;
                m = dp[i][j];
            }
        }
    }

    string ret = "";
    for(; mark1 >= 0 && mark2 >= 0 && s1[mark1] == s2[mark2]; mark1--, mark2--) {
        ret = s1[mark1] + ret;
    }
    return ret;
}

int main() {
    cout << longest_in_both("ABAB", "BABA") << endl;

    return 0;
}