[Solution] add solution power of four

Math/PowerFour.swift

@@ -0,0 +1,29 @@
+/**
+ * Question Link: https://leetcode.com/problems/power-of-four/
+ * Primary idea: n must be a power of 2 and count of zero bits before the (only) set bit is even.
+ * Time Complexity: O(logn), Space Complexity: O(1)
+ */
+
+class PowerOfFour {
+    func isPowerOfFour(_ n: Int) -> Bool {
+        guard n > 0 else {
+            return false
+        }
+
+        let isPowerOfTwo = (n & (n - 1) == 0)
+
+        guard isPowerOfTwo else {
+            return false
+        }
+
+        var numberOfZeros = 0
+        var n = n
+
+        while n != 1 {
+            n = n >> 1
+            numberOfZeros += 1
+        }
+
+        return numberOfZeros % 2 == 0
+    }
+}