Add problem 7 solution

include/p7.h

@@ -0,0 +1,7 @@
+#ifndef P7_H
+#define P7_H
+
+void reverse_string(char* str);
+
+#endif
+

src/main.c

@@ -6,6 +6,7 @@
 #include "p4.h"
 #include "p5.h"
 #include "p6.h"
+#include "p7.h"
 
 void print_int_arr(int* arr, int size) {
 	for (int i = 0; i < size; ++i) {
@@ -67,6 +68,15 @@ int main(void) {
 	str_to_upper(p6_str);
 	printf("\"%s\"\n", p6_str);
 
+	// Problem 7
+	printf("Problem 7:\n");
+	
+	char p7_str[32] = "Hello Exam #2!";
+
+	printf("\"%s\"\n", p7_str);
+	reverse_string(p7_str);
+	printf("\"%s\"\n", p7_str);
+
 	return 0;
 }
 

src/p7.c

@@ -0,0 +1,43 @@
+#include "p7.h"
+
+void reverse_string(char* str) {
+	// First and last character that still need to be swapped
+	char* first = str;
+	char* last = str;
+
+	// Traverse the string until the null character is reached.
+	for (; *last != '\0'; ++last);
+
+	// `last` points to the terminating null character currently,
+	// but we need a pointer to the last character of the string.
+	--last;
+
+	// Traverse string from both ends until the indices meet or cross.
+	//
+	// ex "ABCD:
+	// A     B     C     D
+	// ^                 ^
+	// first             last
+	//
+	// D     B     C     A
+	//       ^     ^
+	//       first last
+	//
+	// D     C     B     A
+	//       ^     ^
+	//       last  first
+	//
+	// At this point the indices have crossed, so we are finished
+	// reversing the string.
+	//
+	while (first < last) {
+		// Swap the first and last characters that need to be swapped.
+		char temp = *first;
+		*first = *last;
+		*last = temp;
+
+		++first;
+		--last;
+	}
+}
+