class Solution {
public:
string longestCommonPrefix(vector<string>& strs) {
string r = strs.size() ? strs[0] : "";
for(auto s: strs){
while(s.substr(0, r.size()) != r){
r = r.substr(0, r.size() - 1);
if(r == "") return r;
}
}
return r;
}
};