String commonPrefixInStringArray(String[] a) { if(a == null || a.length == 0) return null; String commonPrefix = a[0]; for(int i = 1; i< a.length; i++) { commonPrefix = commonPrefixInTwoStrings(b, a[i]); } return commonPrefix; } String commonPrefixInTwoStrings(String a, String b) { if(a == null || b == null) return null; int minL = (a.length < b.length)? a.length : b.length; StringBuilder b = new StringBuilder(); for(int i = 0; i< minL; i++) { if(a.charAt(i) == b.charAt(i)) { b.append(a.charAt(i)); } else { break; } } return b.toString(); }