public int Pop()
{
if (top == null)
{
return(-1);
}
var val = top.value;
top = top.prev;
top.next = null;
return(val);
}
public void Push(int val)
{
if (top == null)
{
top = new stacknode(val);
return;
}
var temp = new stacknode(val);
temp.prev = top;
top.next = temp;
top = temp;
}