slight changes to stackNode struct, void func, etc

bintree.c

@@ -1,4 +1,4 @@
-#include <string.h>
+t #include <string.h>
 #include "stack.h"
 #include "bintree.h"
 

stack.c

@@ -69,7 +69,7 @@ StackNode *pop(StackNode *stack) {
 void *top(StackNode *stack) { return stack != NULL ? stack->data : NULL; }
 
 // Clears stack and releases all memory.
-StackNode *clearStack(StackNode *stack) {
+void *clearStack(StackNode *stack) {
 
   while (stack != NULL) {
 

stack.h

@@ -13,6 +13,7 @@ typedef struct StackNode {
 
   void *data;
   struct StackNode *next;
+  struct StackNode *prev;
   
 } StackNode;