求最大子串长度

int LCS(const string &str1, const string &str2)
{
    int len1 = 0;
    int len2 = 0; //记录字符串长度；
    int longest_len = 0; //记录子串长度；

    len1 = str1.size();
    len2 = str2.size();

    for (int i = 0; i< len1; i++)
    {
        for (int j = 0; j< len2; j++)
        {
            int m = i;
            int n = j;
            int sub_len = 0;

            while (m < len1 && n < len2)
            {
                if （str1[m] != str2[n])
                    break;
                
                m++;
                n++;
                sub_len++;
            }
        }

        if (longest_len < sub_len)
        {
            longest_len = sub_len;
        }
    }

    return longest_len;
}