add problem 50 and test function

Author: supercoderhawk

C/1-50/50-Pow(x, n).c

@@ -0,0 +1,26 @@
+/*
+ * Implement pow(x, n).
+ * Created by supercoderx on 2017/8/15.
+ */
+#include 
+#include 
+
+double myPow(double x, int n) {
+    if (x == 0)
+        return 0;
+    if (n == 0)
+        return 1;
+    int index = n / 2;
+    if (n < 0) {
+        x = 1 / x;
+        index *= -1;
+    }
+    double y = x * x;
+
+    return n % 2 == 0 ? myPow(y, index) : x * myPow(y, index);
+}
+
+void testMyPow() {
+    printf("%lf", myPow(22.14659, -2));
+    printf("%lf", myPow(2.00000, -2147483648));
+}