[Math] add Solution to Power of Two and Power of Three

Author: Yi Gu

Math/PowerThree.swift

@@ -0,0 +1,16 @@
+/**
+ * Question Link: https://leetcode.com/problems/power-of-three/
+ * Primary idea: Use the largest 3^n int number to mod
+ * Time Complexity: O(1), Space Complexity: O(1)
+ *
+ */
+
+ class PowerThree {
+    func isPowerOfThree(n: Int) -> Bool {
+        guard n > 0 else {
+            return false
+        }
+        
+        return 1162261467 % n == 0
+    }
+}

Math/PowerTwo.swift

@@ -0,0 +1,16 @@
+/**
+ * Question Link: https://leetcode.com/problems/power-of-two/
+ * Primary idea: Use and to solve the problem
+ * Time Complexity: O(n), Space Complexity: O(1)
+ *
+ */
+
+class PowerTwo {
+    func isPowerOfTwo(n: Int) -> Bool {
+        guard n > 0 else {
+            return false
+        }
+        
+        return n & (n - 1) == 0
+    }
+}