Zero Minus One Is Huge

#include <cstdio>
#include <vector>

int main() {
    std::vector<int> v{10, 20, 30};

    int n = -1;
    std::puts(n < v.size() ? "A" : "B");            // (1)

    for (int i = v.size() - 1; i >= 0; --i)              // (2)
        std::printf("%d ", v[i]);
    std::putchar('\n');

    std::vector<int> e;
    long total = 0;
    for (std::size_t j = 0; j < e.size() - 1; ++j)      // (3)
        total += e[j];
    std::printf("%ld\n", total);
}

What does this program do?