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:
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        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]);
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It crashed at #42. Then, I modified the last "else branch" into:
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        else if (t) printf("%d\n", a[--t]);
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I got WA at #42.