https://leetcode.com/problems/power-of-four/

powerOfFour.js

/**
 * @param {number} num
 * @return {boolean}
 */

/*
Idea: A power of four is just a non-negative int
with one bit set in at an even index.
So we need to make sure that the given int is:
- non-negative
- has just one bit set
- that bit is at an even index
    - If the bit is at an odd index it's a power of 2,
      but not a power of 4.
*/
// O(1) space and time
var isPowerOfFour = function(num) {
    if(num & (num-1)) return false; // more than one bit set
    if(num <= 0) return false; // negative
    
    const mask = 0xAAAAAAAA >> 1;
    // 0xA = 1010, 0xAA = 10101010 etc
    // Therefore 0xAA... >> 1 = 010101...
    return num & (0xAAAAAAAA >> 1);
};