// stackTriangle.java
// evaluates triangular numbers, stack replaces recursion
// to run this program: C>java StackTriangleApp
import java.io.*;                 // for I/O
////////////////////////////////////////////////////////////////
class Params     // parameters to save on stack
   {
   public int n;
   public int returnAddress;

   public Params(int nn, int ra)
      {
      n=nn;
      returnAddress=ra;
      }
   }  // end class Params
////////////////////////////////////////////////////////////////
class StackX
   {
   private int maxSize;         // size of StackX array
   private Params[] stackArray;
   private int top;             // top of stack
//--------------------------------------------------------------
   public StackX(int s)         // constructor
      {
      maxSize = s;              // set array size
      stackArray = new Params[maxSize];  // create array
      top = -1;                 // no items yet
      }
//--------------------------------------------------------------
   public void push(Params p)   // put item on top of stack
      {
      stackArray[++top] = p;    // increment top, insert item
      }
//--------------------------------------------------------------
   public Params pop()          // take item from top of stack
      {
      return stackArray[top--]; // access item, decrement top
      }
//--------------------------------------------------------------
   public Params peek()         // peek at top of stack
      {
      return stackArray[top];
      }
//--------------------------------------------------------------
   }  // end class StackX
////////////////////////////////////////////////////////////////
class StackTriangleApp
   {
   static int theNumber;
   static int theAnswer;
   static StackX theStack;
   static int codePart;
   static Params theseParams;
//-------------------------------------------------------------
   public static void main(String[] args) throws IOException
      {
      System.out.print("Enter a number: ");
      theNumber = getInt();
      recTriangle();
      System.out.println("Triangle="+theAnswer);
      }  // end main()
//-------------------------------------------------------------
   public static void recTriangle()
      {
      theStack = new StackX(10000);
      codePart = 1;
      while( step() == false)  // call step() until it's true
         ;                     // null statement
      }
//-------------------------------------------------------------
   public static boolean step()
      {
      switch(codePart)
         {
         case 1:                              // initial call
            theseParams = new Params(theNumber, 6);
            theStack.push(theseParams);
            codePart = 2;
            break;
         case 2:                              // method entry
            theseParams = theStack.peek();
            if(theseParams.n == 1)            // test
               {
               theAnswer = 1;
               codePart = 5;   // exit
               }
            else
               codePart = 3;   // recursive call
            break;
         case 3:                              // method call
            Params newParams = new Params(theseParams.n - 1, 4);
            theStack.push(newParams);
            codePart = 2;  // go enter method
            break;
         case 4:                              // calculation
            theseParams = theStack.peek();
            theAnswer = theAnswer + theseParams.n;
            codePart = 5;
            break;
         case 5:                              // method exit
            theseParams = theStack.peek();
            codePart = theseParams.returnAddress; // (4 or 6)
            theStack.pop();
            break;
         case 6:                              // return point
            return true;
         }  // end switch
      return false;
      }  // end triangle
//-------------------------------------------------------------
   public static String getString() throws IOException
      {
      InputStreamReader isr = new InputStreamReader(System.in);
      BufferedReader br = new BufferedReader(isr);
      String s = br.readLine();
      return s;
      }
//-------------------------------------------------------------
   public static int getInt() throws IOException
      {
      String s = getString();
      return Integer.parseInt(s);
      }
//--------------------------------------------------------------
   }  // end class StackTriangleApp
////////////////////////////////////////////////////////////////