diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..2d7008e
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,5 @@
+*doble*
+*.o
+*.exe
+.vscode
+run*Tests
\ No newline at end of file
diff --git a/main.c b/main.c
index 9ede66b..4034716 100644
--- a/main.c
+++ b/main.c
@@ -1,5 +1,6 @@
 #include <stdlib.h>
 #include <stdio.h>
+#include <time.h>
 #include "numbers.h"
 #include "timer.h"
 #include "highscore.h"
@@ -39,6 +40,9 @@ int main(int argc, char *argv[])
 {
     int exitCode = EXIT_FAILURE;
 
+    // set seed
+    srand(time(NULL));
+
     if(argc != 2)
     {
         fprintf(stderr, "Usage: %s <player name>\n", argv[0]);
@@ -83,6 +87,7 @@ int main(int argc, char *argv[])
         saveHighscores(highscorePath);
         clearHighscores();
 
+        free(numbers);
         exitCode = EXIT_SUCCESS;
     }
 
diff --git a/makefile b/makefile
index 1f15f75..5654302 100644
--- a/makefile
+++ b/makefile
@@ -1,5 +1,5 @@
 CC = gcc
-FLAGS = -g -Wall -lm
+CFLAGS = -g -Wall -lm
 
 ifeq ($(OS),Windows_NT)
 	include makefile_windows.variables
@@ -27,16 +27,29 @@ doble_initial:
 program_obj_files = stack.o bintree.o numbers.o timer.o highscore.o
 
 doble : main.o $(program_obj_files)
-	$(CC) $(FLAGS) $^ -o doble
+	$(CC) $(CFLAGS) $^ -o doble
 
-$(program_obj_filesobj_files): %.o: %.c
-	$(CC) -c $(FLAGS) $^ -o $@
+$(program_obj_files): %.o: %.c
+	$(CC) -c $(CFLAGS) $^ -o $@
 
 # --------------------------
 # Unit Tests
 # --------------------------
-unitTests:
-	echo "needs to be implemented"
+TEST_STACK_SOURCES = stack.c test_stack.c $(unityfolder)/unity.c
+TEST_BINTREE_SOURCES = bintree.c test_bintree.c stack.c $(unityfolder)/unity.c
+TEST_NUMBERS_SOURCES = stack.c numbers.c bintree.c $(unityfolder)/unity.c test_numbers.c
+
+stackTests: $(TEST_STACK_SOURCES) stack.h
+	$(CC) $(CFLAGS) -I$(unityfolder) $(TEST_STACK_SOURCES) -o runStackTests
+	./runStackTests
+
+bintreeTests: $(TEST_BINTREE_SOURCES) stack.h bintree.h
+	$(CC) $(CFLAGS) -I$(unityfolder) $(TEST_BINTREE_SOURCES) -o runBintreeTests
+	./runBintreeTests
+
+numbersTests: $(TEST_NUMBERS_SOURCES) stack.h bintree.h numbers.h
+	$(CC) $(CFLAGS) -I$(unityfolder) $(TEST_NUMBERS_SOURCES) -o runNumbersTests
+	./runNumbersTests
 
 # --------------------------
 # Clean
diff --git a/stack.c b/stack.c
index e3a90d4..21bc819 100644
--- a/stack.c
+++ b/stack.c
@@ -1,33 +1,55 @@
 #include <stdlib.h>
 #include "stack.h"
 
-//TODO: grundlegende Stackfunktionen implementieren:
+// TODO: grundlegende Stackfunktionen implementieren:
 /*  * `push`: legt ein Element oben auf den Stack,
-    * `pop`: entfernt das oberste Element,
-    * `top`: liefert das oberste Element zurück,
-    * `clearStack`: gibt den gesamten Speicher frei. */
+ * `pop`: entfernt das oberste Element,
+ * `top`: liefert das oberste Element zurück,
+ * `clearStack`: gibt den gesamten Speicher frei. */
 
 // Pushes data as pointer onto the stack.
 StackNode *push(StackNode *stack, void *data)
 {
+    // this is the new top node
+    StackNode *newTopNode = malloc(sizeof(StackNode));
 
+    if (newTopNode == NULL)
+    {
+        return NULL;
+    }
+
+    newTopNode->data = data;
+    newTopNode->next = stack;
+
+    return newTopNode;
 }
 
 // Deletes the top element of the stack (latest added element) and releases its memory. (Pointer to data has to be
 // freed by caller.)
 StackNode *pop(StackNode *stack)
 {
-
+    if (!stack)
+    {
+        return NULL;
+    }
+    StackNode *nextNode = stack->next;
+    free(stack);
+    return nextNode;
 }
 
 // Returns the data of the top element.
 void *top(StackNode *stack)
 {
-
+    if (!stack)
+    {
+        return NULL;
+    }
+    return stack->data;
 }
 
 // Clears stack and releases all memory.
 void clearStack(StackNode *stack)
 {
-
+    while (pop(stack))
+        ;
 }
\ No newline at end of file
diff --git a/stack.h b/stack.h
index f7d542d..83dc6be 100644
--- a/stack.h
+++ b/stack.h
@@ -1,13 +1,17 @@
 #ifndef STACK_H
 #define STACK_H
 
-/* A stack is a special type of queue which uses the LIFO (last in, first out) principle. 
-This means that with each new element all other elements are pushed deeper into the stack. 
+/* A stack is a special type of queue which uses the LIFO (last in, first out) principle.
+This means that with each new element all other elements are pushed deeper into the stack.
 The latest element is taken from the stack. */
 
 #include <stdlib.h>
 
-//TODO: passenden Datentyp als struct anlegen
+typedef struct StackNode
+{
+    struct StackNode *next;
+    void *data;
+} StackNode;
 
 // Pushes data as pointer onto the stack.
 StackNode *push(StackNode *stack, void *data);
diff --git a/test_bintree.c b/test_bintree.c
new file mode 100644
index 0000000..3ca280d
--- /dev/null
+++ b/test_bintree.c
@@ -0,0 +1,39 @@
+#include "unity.h"
+#include "bintree.h"
+#include "string.h"
+
+void setUp(void)
+{
+    // set stuff up here
+}
+
+void tearDown(void)
+{
+    // set stuff up here
+}
+
+// this adds some strings and checks if they are returned in the right order
+void test_insert_and_retrieve(void)
+{
+    char *data1 = "a_this";
+    char *data2 = "b_is";
+    char *data3 = "c_testdata";
+
+    TreeNode *root = addToTree(NULL, data1, strlen(data1) + 1, (CompareFctType)&strcmp, NULL);
+    addToTree(root, data2, strlen(data2) + 1, (CompareFctType)&strcmp, NULL);
+    addToTree(root, data3, strlen(data3) + 1, (CompareFctType)&strcmp, NULL);
+
+    TEST_ASSERT_EQUAL_STRING(data1, (char *)nextTreeData(root));
+    TEST_ASSERT_EQUAL_STRING(data2, (char *)nextTreeData(NULL));
+    TEST_ASSERT_EQUAL_STRING(data3, (char *)nextTreeData(NULL));
+
+    clearTree(root);
+}
+
+int main(void)
+{
+    printf("============================\nBintree tests\n============================\n");
+    UNITY_BEGIN();
+    RUN_TEST(test_insert_and_retrieve);
+    return UNITY_END();
+}
\ No newline at end of file
diff --git a/test_numbers.c b/test_numbers.c
new file mode 100644
index 0000000..a7e6872
--- /dev/null
+++ b/test_numbers.c
@@ -0,0 +1,50 @@
+#include "unity.h"
+// #include "bintree.h"
+// #include "string.h"
+#include "numbers.h"
+#include "stdlib.h"
+
+void setUp(void)
+{
+    // set stuff up here
+}
+
+void tearDown(void)
+{
+    // set stuff up here
+}
+
+// getDuplicate on array without duplicats
+// expects 0/error
+void test_get_duplicate_error(void)
+{
+    unsigned int input[] = {1, 5, 9, 2, 4};
+    unsigned int len = sizeof(input) / sizeof(input[0]);
+
+    TEST_ASSERT_EQUAL_UINT(0, getDuplicate(input, len));
+}
+
+// this tries to brute force a triple
+void test_for_triple(void)
+{
+    // this test is less effective if srand is called inside createNumbers()
+    for (int i = 0; i < 100000; i++)
+    {
+        unsigned int *numbers = createNumbers(3);
+        if (numbers[0] == numbers[1] && numbers[1] == numbers[2])
+        {
+            // fail the test
+            TEST_ASSERT(0);
+        }
+        free(numbers);
+    }
+}
+
+int main(void)
+{
+    printf("============================\nNumbers tests\n============================\n");
+    UNITY_BEGIN();
+    RUN_TEST(test_get_duplicate_error);
+    RUN_TEST(test_for_triple);
+    return UNITY_END();
+}
\ No newline at end of file
diff --git a/test_stack.c b/test_stack.c
new file mode 100644
index 0000000..e9843ab
--- /dev/null
+++ b/test_stack.c
@@ -0,0 +1,47 @@
+#include "unity.h"
+#include "stack.h"
+
+int data1 = 10;
+int data2 = 20;
+int data3 = 30;
+
+StackNode *stack = NULL;
+
+void setUp(void)
+{
+    // set stuff up here
+}
+
+void tearDown(void)
+{
+    clearStack(stack);
+}
+
+void test_push_and_pop(void)
+{
+    stack = push(stack, &data1);
+    stack = push(stack, &data2);
+    stack = push(stack, &data3);
+
+    TEST_ASSERT_EQUAL_PTR(top(stack), &data3);
+    stack = pop(stack);
+    TEST_ASSERT_EQUAL_PTR(top(stack), &data2);
+    stack = pop(stack);
+    TEST_ASSERT_EQUAL_PTR(top(stack), &data1);
+    stack = pop(stack);
+}
+
+void test_handle_NULL(void)
+{
+    TEST_ASSERT_NULL(pop(stack));
+    TEST_ASSERT_NULL(top(stack));
+}
+
+int main(void)
+{
+    printf("============================\nStack tests\n============================\n");
+    UNITY_BEGIN();
+    RUN_TEST(test_push_and_pop);
+    RUN_TEST(test_handle_NULL);
+    return UNITY_END();
+}
\ No newline at end of file
diff --git a/timer.c b/timer.c
index fd8f6c1..2c80f33 100644
--- a/timer.c
+++ b/timer.c
@@ -1,6 +1,12 @@
 #include "timer.h"
 
-#if __APPLE__
+#ifdef __linux__
+// Defines strict posix compliance for CLOCK_MONOTONIC
+#define _POSIX_C_SOURCE 199309L
+#include <time.h>
+#endif
+
+#if __APPLE__ || __linux__
 #include <sys/time.h>
 static struct timespec start = {0, 0};
 
@@ -14,14 +20,15 @@ void startTimer()
 double stopTimer()
 {
     struct timespec end;
-    
+
     clock_gettime(CLOCK_MONOTONIC, &end);
 
     unsigned long long delta_us = (end.tv_sec - start.tv_sec) * 1000000 + (end.tv_nsec - start.tv_nsec) / 1000;
 
     double measuredSeconds = (double)delta_us / 1000000.;
 
-    if(start.tv_nsec > 0) {
+    if (start.tv_nsec > 0)
+    {
         start.tv_nsec = 0;
         start.tv_sec = 0;
     }
@@ -45,7 +52,7 @@ double stopTimer()
 {
     double measuredSeconds = (clock() - (double)startClocks) / CLOCKS_PER_SEC;
 
-    if(startClocks > 0)
+    if (startClocks > 0)
         startClocks = 0;
     else
         measuredSeconds = -1;