342. Power of Four

Problem:

Given an integer (signed 32 bits), write a function to check whether it is a power of 4.

Example 1:

Input: 16
Output: true

Example 2:

Input: 5
Output: false

Follow up: Could you solve it without loops/recursion?

思路：

Solution (C++):

bool isPowerOfFour(int num) {
    if (num <= 0)  return false;
    while (num%4 == 0)  num /= 4;
    if (num == 1)  return true;
    else return false;
}

性能：

Runtime: 4 ms  Memory Usage: 6 MB

思路：

Solution (C++):

bool isPowerOfFour(int num) {
    return (num > 0) && (num & (num-1))  == 0 && (num-1)%3 == 0;
}

性能：

Runtime: 0 ms  Memory Usage: 5.9 MB