Array.prototype.check = function() {
  var upper = Number.MAX_VALUE;
  var lower = Number.MIN_VALUE;
  for (var i = 0; i < this.length; i++) {
    if ( (upper <= this[i]) || (this[i] <= lower) )
      return false;
    if (this[i] < this[i+1])
      lower = this[i];
    else
      upper = this[i];
  }
  return true;
};