Write a function to find the longest common prefix string amongst an array of strings.
Input: "edwardshi", "edward", "edwar", "edwardshidd"
Output: "edwar"
public static String longestCommonPrefix(String s[]){
if(s==null || s.length == 0) return null;
String res = s[0];
for(int i=1; i<s.length; i++){
while(s[i].indexOf(res, 0) != 0){
res = res.substring(0, res.length() - 1);
}
}
return res;
}