Determine whether a given integer is a power of four. A power of four is a positive integer of the form 4 raised to some non-negative exponent (1, 4, 16, 64 and so on). Return false for zero, negative numbers, and null.
Examples:
Input: n = 16
Output: true
Explanation: 16 equals 4 raised to the power 2
Input: n = 1
Output: true
Explanation: 1 equals 4 raised to the power 0
Input: n = 5
Output: false
Explanation: 5 is not a power of four
Input: n = 8
Output: false
Explanation: 8 is a power of two but not a power of four
Input: n = null
Output: false
Explanation: Null input returns false
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