Power of Four

Problem

Return true iff n is a power of four (1, 4, 16, 64, …).

Constraints

• −2³¹ ≤ n ≤ 2³¹−1

Approach — Core Patterns

This is a Core Patterns problem. The idea: pick the data structure best matched to the constraints and solve it in one clean pass. Work through the reference code below line by line, then re-derive it yourself in the editor — that's how the pattern sticks.

Complexity: see the walkthrough below.

Solution code

Python

class Solution:
    def isPowerOfFour(self, n):
        return n > 0 and (n & (n - 1)) == 0 and (n & 0x55555555) != 0

Java

class Solution {
    public boolean isPowerOfFour(int n) {
        return n > 0 && (n & (n - 1)) == 0 && (n & 0x55555555) != 0;
    }
}

Practice it

Reading a solution isn't the same as being able to write it under pressure. Open this problem in the in-browser editor, hide the solution, and solve it from scratch — your code runs against real test cases instantly.