#include <string>
using std::string;
int LongestCommonSubstring(const string& str1, const string& str2)
{
if(str1.empty() || str2.empty())
{
return 0;
}
int *curr = new int [str2.size()];
int *prev = new int [str2.size()];
int *swap = nullptr;
int maxSubstr = 0;
for(int i = 0; i<str1.size(); ++i)
{
for(int j = 0; j<str2.size(); ++j)
{
if(str1[i] != str2[j])
{
curr[j] = 0;
}
else
{
if(i == 0 || j == 0)
{
curr[j] = 1;
}
else
{
curr[j] = 1 + prev[j-1];
}
//The next if can be replaced with:
//maxSubstr = max(maxSubstr, curr[j]);
//(You need algorithm.h library for using max())
if(maxSubstr < curr[j])
{
maxSubstr = curr[j];
}
}
}
swap=curr;
curr=prev;
prev=swap;
}
delete [] curr;
delete [] prev;
return maxSubstr;
}

SOURCE