diff --git a/bintree.c b/bintree.c
index 5cf82a9..0c5b453 100644
--- a/bintree.c
+++ b/bintree.c
@@ -1,4 +1,4 @@
-#include <string.h>
+t #include <string.h>
 #include "stack.h"
 #include "bintree.h"
 
diff --git a/stack.c b/stack.c
index 35e7708..dbda947 100644
--- a/stack.c
+++ b/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) {
 
diff --git a/stack.h b/stack.h
index f90d217..fb1cd04 100644
--- a/stack.h
+++ b/stack.h
@@ -13,7 +13,8 @@ typedef struct StackNode {
 
   void *data;
   struct StackNode *next;
-
+  struct StackNode *prev;
+  
 } StackNode;
 
 StackNode *createNode(void *data);