class Solution {
public:
    string longestCommonPrefix(vector<string>& strs) { // binary search can be used.

        int n = strs.size();

        int ans = 1;
        if(strs.empty())return "";


        for(int ans = 1;true;ans++){

        for(string s:strs){

            if(s.length() < ans || s[ans-1]!=strs[0][ans-1]){
                return s.substr(0,ans-1);
            }
        }

        }
    }
};