code: public static String commonSuffix(final String a, final String b) {
          if (N.isNullOrEmpty(a) || N.isNullOrEmpty(b)) {
              return N.EMPTY_STRING;
          }
  
          final int aLength = a.length();
          final int bLength = b.length();
          int maxSuffixLength = Math.min(aLength, bLength);
          int s = 0;
  
          while (s < maxSuffixLength && a.charAt(aLength - s - 1) == b.charAt(bLength - s - 1)) {
              s++;
          }
  
          if (validSurrogatePairAt(a, aLength - s - 1) || validSurrogatePairAt(b, bLength - s - 1)) {
              s--;
          }
  
          if (s == aLength) {
              return a.toString();
          } else if (s == bLength) {
              return b.toString();
          } else {
              return a.subSequence(aLength - s, aLength).toString();
          }
      }
docs: Note: copy rights: Google Guava.
  
  Returns the longest string {@code suffix} such that
  {@code a.toString().endsWith(suffix) && b.toString().endsWith(suffix)},
  taking care not to split surrogate pairs. If {@code a} and {@code b} have
  no common suffix, returns the empty string.