Push
by Isai Damier, Android Engineer @ Google

/**************************************************************************
 * Author: Isai Damier
 * Title: Stack With Get-Min
 * Project: geekviewpoint
 * Package: datastructure
 *
 * Description: Normally, only the top of a stack is available for
 *    peeking and popping. Here, however, the requirement is to make
 *    the minimum value on the stack always viewable in O(1).
 *    To that end, in addition to the primary stack of operation
 *    (called primary), a separate stack (called minimums) must be
 *    used to track the minimum value on the primary stack. The point
 *    is not to pop the minimum value in O(1); that would require a
 *    bit more work. Rather, the getMinimum function allows peeking
 *    into the stack, as it were, to see the minimum value on the
 *    stack.
 *
 *    Throughout the implementation it should be clear that the
 *    primary stack is the real (i.e. representative) stack and that
 *    the minimums stack is an auxiliary stack that exists only so
 *    the getMinimum() function may exist.
 *************************************************************************/ 
 import java.util.Stack;

public class StackWithGetMin {

  /**
   *  Stack<Integer> primary: where all the elements actually reside.
   */
  private final Stack<Integer> primary = new Stack<Integer>();
  /**
   *  Stack<Integer> minimums: used to track the minimum values on
   *  the stack. The most minimum value is always on top.
   */
  private final Stack<Integer> minimums = new Stack<Integer>();

  /**********************************************************************
   * Statement:
   *   Add the given element to the stack.
   *
   * Time Complexity of Solution:
   *   Best = Average = Worst = const.
   *
   * Technical Details: If the stack is empty, then the given element
   *     is perforce also the minimum element on the stack. Therefore
   *     mark the element as minimum by also pushing it into the
   *     minimums stack. On the other hand, if the stack (i.e. the
   *     primary stack) is not empty, check if the new element is less
   *     than the current minimum element. If it is, make the new
   *     element the new minimum.
   *
   *     The explanation is slightly different from the actual code
   *     to give the reader a different viewpoint.
   *
   **********************************************************************/
  public void push(int e) {
    if (primary.isEmpty() || e < minimums.peek()) {
      minimums.push(e);
    }
    primary.push(e);
  }
}

import org.junit.Test;
import static org.junit.Assert.*;

public class StackWithGetMinTest {

  /**
   * Test of push method, of class StackWithGetMin.
   */
  @Test
  public void testPush() {
    System.out.println("push");
    int e = 0;
    StackWithGetMin stack = new StackWithGetMin();
    Integer[] inputs = {40, 65, 5, 6, 32, 4, 7, 1, 2, 84, 9, 10};
    for (int i : inputs) {
      stack.push(i);
    }
    assertFalse(stack.isEmpty());
    assertEquals(inputs.length, stack.size());
  }
}