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

1 public class Solution { 2     public String longestCommonPrefix(String[] strs) { 3         if (strs == null || strs.length == 0) { 4             return ""; 5         } else if (strs.length < 2) { 6             return strs[0]; 7         } 8  9         String r = strs[0];10         int index = r.length();11         for (int i = 1; i < strs.length; i++) {12             int j = 0;13             for (; j < strs[i].length() && j < index; j++) {14                 if (strs[i].charAt(j) != r.charAt(j)) {15                     break;16                 }17             }18             if (j == 0) {19                 return "";20             }21             index = j;22         }23 24         return r.substring(0, index);25     }26 }