public class StackManager {
  private int[] memory;
  // the memory we manage
  private int top;
  // index of top stack block

  /**
   * StackManager constructor.
   * @param initialMemory int[] of memory to manage
   */
  public StackManager(int[] initialMemory) {
    memory = initialMemory;
    top = memory.length;
  }

  /**
   * Allocate a block and return its address.
   * @param requestSize int size of block, > 0
   * @return block address
   * @throws StackOverflowError if out of stack space
   */
  public int push(int requestSize) {
    int oldtop = top;
    top -= (requestSize+1);
    // extra word for oldtop
    if (top<0) throw new StackOverflowError();
    memory[top] = oldtop;
    return top+1;
  }

  /**
   * Pop the top stack frame.  This works only if the
   * stack is not empty.
   */
  public void pop() {
    top = memory[top];
  }
}