add problem 7 add test function

Author: supercoderhawk

C/1-50/7-Reverse-Integer.c

@@ -0,0 +1,33 @@
+/*
+ * Reverse digits of an integer.
+ *
+ * Example1: x = 123, return 321
+ * Example2: x = -123, return -321
+ * Created by supercoderhawk on 2017/7/23.
+ */
+
+#include 
+#include 
+
+int reverse(int x) {
+    long long maxInt = (long) pow(2, 31), result = 0;
+    int num = x, length = 0;
+    while (num != 0) {
+        num /= 10;
+        length += 1;
+    }
+    num = x;
+    for (int i = length - 1; i >= 0; i--) {
+        result = result * 10 + (num % 10);
+        num /= 10;
+    }
+    if (result > maxInt || result < -maxInt - 1)
+        result = 0;
+
+    return (int) result;
+}
+
+void testReverse() {
+    printf("%d", reverse(123));
+    printf("%d", reverse(-2147483648));
+}