#include <iostream>

char *largestCommonSubstr(const char s1[], const char s2[]);

using namespace std;
int main(int argc, const char *argv[])
{

    const char *s1[] = {"Boston", "James", "computer" , "abc", "software"};
    const char *s2[] = {"Killington", "Thames", "Counter", "abba", "MICROSOFT"};

    for (int i = 0; i < 5; i++) {
        char *substr = largestCommonSubstr(s1[i], s2[i]);
        if (substr)
            cout << "longest common substring of " << s1[i] << " and " << s2[i] << " is " << substr << endl;
        else
            cout << "no common substrings between " << s1[i] << " and " << s2[i] << endl;
    }

    return 0;
}

char *largestCommonSubstr(const char s1[], const char s2[])
{
    int len = 0;
    int longest = 0;
    int longestpos;
    int pos1,pos2,ofst; 

    for (pos1 = 0; s1[pos1]; pos1++) {
        for (pos2 = 0; s2[pos2]; pos2++) {
            for (ofst = 0,len=0; s1[pos1+ofst] == s2[pos2+ofst++]; len++) {}
            if (len > longest) {
                longest = len;
                longestpos = pos1;
            }
        }
    }
    
    char *result = longest != 0 ? new char(longest+1) : NULL;
    if (result) {
        for (int i = 0; i < longest; i++) {
            result[i] = s1[longestpos+i];
        }
        result[longest] = '\0';
    }
    return result;
}