Number of moments where all turned on bulbs are shined [closed]

Answer 1

You could do it in O(n):

public int solution(int[] a) {
    Set missing = new HashSet<>();
    Set store = new HashSet<>();
    int count = 0;
    for (int i = 0; i < a.length; i++) {
        if (!store.contains(i + 1) && i + 1 != a[i])
            missing.add(i + 1);
        if (i + 1 < a[i])
            store.add(a[i]);
        else
            missing.remove(a[i]);
        if (missing.isEmpty())
            count++;
    }
    return count;
}