add semaphore solution

exercises/semaphore/semaphore.c

 #include "semaphore.h"
 
 void sem_init(semaphore *sem, int count) {
-    // TODO: implement this function
+    sem->count = count;
+    pthread_mutex_init(&sem->mutex, NULL);
+    pthread_cond_init(&sem->condition, NULL);
 }
 
 void sem_wait(semaphore *sem) {
-    // TODO: implement this function
+    pthread_mutex_lock(&sem->mutex);
+    while (sem->count <= 0) {
+        pthread_cond_wait(&sem->condition, &sem->mutex);
+    }
+    sem->count--;
+    pthread_mutex_unlock(&sem->mutex);
 }
 
 void sem_post(semaphore *sem) {
-    // TODO: implement this function
+    pthread_mutex_lock(&sem->mutex);
+    sem->count++;
+    pthread_cond_signal(&sem->condition);
+    pthread_mutex_unlock(&sem->mutex);
 }
 
 void sem_destroy(semaphore *sem) {
-    // TODO: implement this function
+    pthread_mutex_destroy(&sem->mutex);
+    pthread_cond_destroy(&sem->condition);
 }