14. Longest Common Prefix

class Solution {public:    //求两个字符串的最长公共前缀    string longestCommonPrefix_twoString(string s1, string s2)    {        string ret;        int l = s1.size() < s2.size() ? s1.size() : s2.size();         for(int i = 0; i < l; i++)        {            if(s1.at(i) == s2.at(i))                ret.push_back(s1.at(i));            else                break;        }        return ret;    }        string longestCommonPrefix(vector<string>& strs)     {        string ret;        int l = strs.size();        if(l == 0)            return ret;        if(l == 1)            return strs.at(0);                ret = longestCommonPrefix_twoString(strs.at(0), strs.at(1));        for(int i = 2; i < l; i++)            ret = longestCommonPrefix_twoString(ret, strs.at(i));        return ret;    }};