function lcs(a, b) {
  var aSub = a.substr(0, a.length-1);
  var bSub = b.substr(0, b.length-1);

  if (a.length == 0 || b.length == 0) {
    return "";
  } else if (a.charAt(a.length-1) == b.charAt(b.length-1)) {
    return lcs(aSub, bSub) + a.charAt(a.length-1);
  } else {
    var x = lcs(a, bSub);
    var y = lcs(aSub, b);
    return (x.length > y.length) ? x : y;
  }
}