[Math] add Solution to Pow(x, n)

Math/Pow.swift

@@ -0,0 +1,43 @@
+/**
+ * Question Link: https://leetcode.com/problems/powx-n/
+ * Primary idea: Classic Recursion and handle positive/negative case at first
+ *
+ * Time Complexity: O(logn), Space Complexity: O(1)
+ */
+
+class Pow {
+    func myPow(x: Double, _ n: Int) -> Double {
+        guard n != 0 else {
+            return 1
+        }
+        guard x != 0 else {
+            return 0
+        }
+
+        var res = _helper(abs(x), abs(n))
+
+        if n < 0 {
+            res = 1 / res
+        }
+        if n % 2 != 0 && x < 0 {
+            res = -res
+        }
+
+        return res
+    }
+
+    private func _helper(x: Double, _ n: Int) -> Double {
+        guard n != 0 else {
+            return 1
+        }
+        guard n != 1 else {
+            return x
+        }
+
+        if n % 2 == 0 {
+            return _helper(x * x, n / 2)
+        } else {
+            return _helper(x, n - 1) * x
+        }
+    }
+}