Write a function to find the longest common prefix string amongst an array of strings.

1 class Solution { 2 public: 3     string longestCommonPrefix(vector
     
       &strs) { 4         string res; 5         if(strs.empty())  6             return res; 7         for(int i = 0; i < strs[0].size(); i++) { 8             char ch = strs[0][i]; 9             for(int j = 1; j < strs.size(); j++) {10                 if(strs[j][i] != ch || i == strs[j].size()) {11                     return res;12                 }13             }14             res.push_back(ch);15         }16     }17 };