@Override
   public boolean equals (Object o) {
      // Return false if stack sizes are different.
      if (((DoubleStack) o).doubleStack.size() != this.doubleStack.size()) {
         return false;
      }

      // Start comparing both DoubleStack one by one. Return false if mismatch is found.
      for (int i = 0; i < this.doubleStack.size(); i++) {
         if (((DoubleStack) o).doubleStack.get(i) != this.doubleStack.get(i)) {
            return false;
         }
      }
      // Return true if no mismatch is found.
      return true;
   }