Apex class
-------------
public class StringStack {

      private List<String> stack;

    public StringStack()
    {
         stack = new List<String>();

    }

    public void push(String s)
    { 
        stack.add(s); 
    }
    public String pop() 
    {
       
    return stack.remove( lastItemIndex );
    }
    public String peak() 
    {

        return stack.get( lastItemIndex );

    }

 
    public Boolean isEmpty() 
    { 
        return stack.isEmpty();
    }
     

    // Helper Property

    private Integer lastItemIndex
    {
        get 
        { 
            return stack.size() - 1;
            }
    }

}



Test clas
------------
@isTest
public class TestStringStack 
 {
public static testmethod void StringStackcheck()
{
    //Instantiate a StringStack.

    StringStack stack = new StringStack();
 // Set up some test data.

    String bottomString = 'Bottom String';
    String middleString = 'Middle String';
    String topString = 'Top String';

      // Call the push() method with multiple objects

    stack.push(bottomString);

    stack.push(middleString);
    stack.push(topString);

    // Verify that the 'top' object is the object we expected

    String peakValue = stack.peak();

    System.assertEquals(topString, peakValue);

    // Verify that the order of the objects is as we expected
    String popValue = stack.pop();
    System.assertEquals(topString, popValue);
    popValue = stack.pop();
    System.assertEquals(middleString, popValue);
    popValue = stack.pop();
    System.assertEquals(bottomString, popValue);
    System.assert(stack.isEmpty());

}
 }