/* 0 means door is closed, 1 means door is open */
for (i = 0; i < 100; i++) {
    for (j = i; j < 100; j += (i + 1)) {
        d[j] = 1 - d[j]     /* Toggle door */
    }
}

"Open doors:
"
for (i = 0; i < 100; i++) {
    if (d[i] == 1) (i + 1)
}