Solve Exercise 4-13: Recursive reverse()

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diff --git a/ch4/4-13_recursive-reverse.c b/ch4/4-13_recursive-reverse.c
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+#include <stdio.h>
+#include <string.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 4-13: Write a recursive version of the function reverse(s), which
+ * reverses the string `s` in place.
+ *
+ * Answer: Much like the last exercise, the trick is in terminating your flow
+ * early and resetting state for the next string.
+ */
+
+void reverse(char[]);
+
+int main() {
+	char foo[40] = "Learning C is fun and challenging.";
+	printf("Before: %s\n", foo);
+	reverse(foo);
+	printf(" After: %s\n", foo);
+	/* I don't know how to set a char array to another string... */
+	char bar[30] = "Hello out there!";
+	printf("Before: %s\n", bar);
+	reverse(bar);
+	printf(" After: %s\n", bar);
+	return 0;
+}
+
+void reverse(char s[]) {
+	static int i = 0;
+	static int j = 0;
+	int c;
+
+	if (j == 0) {
+		j = strlen(s) - 1;
+	}
+
+	if (i < j) {
+		c = s[i];
+		s[i] = s[j];
+		s[j] = c;
+		i++;
+		j--;
+		reverse(s);
+		return;
+	}
+	i = 0;
+	j = 0;
+}