Question

I have two strings the ending substring of first is the starting substring of the second,ex

string left : ONESTRING
string right : STRINGTWO

I have to merge them so that the resulting string is

result string : ONESTRINGTWO

The length of the common substring is not known in advance.
If the starting and ending strings are not common, i need to return the concatenation of the strings.

This is what i am doing currently.

for(int i = 1;i< left.length();i++) {
        //substring of length "i" from last of left string
        string temp = left.substr(left.length() -1 -i,i);
        if(temp.length() < right.length()) {
            //check if the right string starts with the above substring 
            if (strncmp(right.c_str(), temp.c_str(), strlen(temp.c_str())) == 0 ) {
                // common substring found, save this result 
                found =  true;
                result = left.substr(0,left.length()-i-1) + right;
            }

        }
    }

if(found == true) {
    return result;
} else {
    return left + right;
}

I would be thankful for any pointers to a simpler implementation (in any language ).

Answer 1

By smart use of pointer arithmetic, you can skip the calls to substr (which do allocation) and strlen (which take O(n) time in the length of the string).

std::string concat(std::string const &left, std::string const &right)
{
    size_t n = left.length();
    for (size_t i=0; i<n; i++)
        if (std::strncmp(left.c_str() + i, right.c_str(), n - i) == 0)
            return left + (right.c_str() + n - i);

    return left + right;
}