Longest Common Prefix leetcode 14

class Solution {
public:
string strPrefix(string s1,string s2){
if(s1.length()==0||s2.length()==0)return "";
int len=s1.length()<s2.length()?s1.length():s2.length();
int i;
for(i=0;i<len;i++){
if(s1[i]!=s2[i])break;
}
return s1.substr(0,i);

}

string longestCommonPrefix(vector<string>& strs) {
if(strs.size()==0)return "";
string str=strs[0];
for(int i=1;i<strs.size();i++){
str=strPrefix(str,strs[i]);
}
return str;
}
};