# O(1) Java bit operation

• Java O(1) solution:

Explain:

• for n > 3, result is same as n = 3, if same series of operations apply on 3 bulbs generate k distinct result, apply them on more than 3 bulbs would also generate k distinct result.
• only even and odd matters for each operation. So the total possible op series would be [0,0,0,0], [0,0,0,1]...to [1,1,1,1], 16 total
• order of operations doesn't matter
``````public int flipLights(int n, int m) {
int odd = 0b101;
int even = 0b010;
int three = 0b001;
if (n > 3) n = 3;
Set<Integer> res = new HashSet<>();
for (int i = 0; i < 16; ++i) {
int cnt = 0;

for (int k = 0; k < 4; ++k) if ((i&(1<<k)) != 0)++cnt;
if (cnt % 2 != m % 2 || m < cnt) continue;
int v = (1 << n) - 1;
int mask = v;
if ((i & 1) != 0) v = ~v;
if ((i & 2) != 0) v = even&~v|v&~even;
if ((i & 4) != 0) v = odd&~v|v&~odd;
if ((i & 8) != 0) v = three&~v|v&~three;
res.add(v&mask);
}
return res.size();
}
``````

• This post is deleted!

• @donggua_fu add more details and replace the magic constants with binary literals

• @feng.peng.3597 enen, I almost got the idea..Thank you so much~~(^_^)

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