The problem say that "It is guaranteed that during the experiment each time a coin was to be removed the stack was not empty."
But when I submit such a brute force code:
--------------------
        if (x > 0) a[t++] = x;
        else if (x == 0) {
            if (t + t < maxn) {
                copy(a, a + t, a + t);
                t += t;
            }
        }
        else printf("%d\n", a[--t]);
--------------------
It crashed at #42. Then, I modified the last "else branch" into:
--------------------
        else if (t) printf("%d\n", a[--t]);
--------------------
I got WA at #42.

What if t = maxn/2 + 1 and than t + 1 pop operations?
For example:
1000000
1
0
0
..
0 (20 times)
-1
-1
..
-1 (1000000 - 21 times)

You will store only 2^19 copies in array, and get crash

Oh..Sorry..It's my fault.
I only set the size of a to 1000001 * 2, but I forgot to modify the maxn....
I apologize for asking so stupid question.T_T
Accepted now, thank you very much.

Yeah,I've made the same mistake,and thank you so much!