public void SetOfStacksTest()
{
var s = new SetOfStacks();
Assert.Equal(1, s.StacksCount);
s.Push(1);
s.Push(2);
s.Push(3);
s.Push(4);
Assert.Equal(2, s.StacksCount);
Assert.Equal(4, s.Pop());
Assert.Equal(3, s.Pop());
Assert.Equal(2, s.Pop());
Assert.Equal(1, s.Pop());
Assert.Throws <InvalidOperationException>(() => s.Pop());
s.Push(1); // stack 0
s.Push(2);
s.Push(3);
s.Push(4); // stack 1
s.Push(5);
s.Push(6);
s.Push(7); // stack 2
Assert.Equal(6, s.PopAt(1));
Assert.Equal(5, s.PopAt(1));
Assert.Equal(4, s.PopAt(1));
Assert.Throws <InvalidOperationException>(() => s.PopAt(1));
Assert.Throws <ArgumentOutOfRangeException>(() => s.PopAt(3));
Assert.Throws <ArgumentOutOfRangeException>(() => s.PopAt(-1));
Assert.Equal(3, s.StacksCount);
// for follow up use Pop2
Assert.Equal(7, s.Pop2());
Assert.Equal(3, s.Pop2());
Assert.Equal(1, s.StacksCount);
}