Proper returns

src/tree.c

@@ -43,6 +43,8 @@ int tree_insert(Tree *tree, int data) {
       }
     }
   }
+
+  return 0;
 }
 
 bool tree_contains(Node *node, int data) {
@@ -59,6 +61,8 @@ int tree_remove(Node **node, int data) {
   if (node == NULL)
     return false;
   // TODO
+
+  return 0;
 }
 
 int tree_clear(Node *node) {