diff --git a/semester3/exercises/exercise2.13.c b/semester3/exercises/2-Algorighm-Analysis/exercise2.13.c
similarity index 100%
rename from semester3/exercises/exercise2.13.c
rename to semester3/exercises/2-Algorighm-Analysis/exercise2.13.c
diff --git a/semester3/exercises/exercise2.6.c b/semester3/exercises/2-Algorighm-Analysis/exercise2.6.c
similarity index 100%
rename from semester3/exercises/exercise2.6.c
rename to semester3/exercises/2-Algorighm-Analysis/exercise2.6.c
diff --git a/semester3/exercises/homework2.c b/semester3/exercises/2-Algorighm-Analysis/homework2.c
similarity index 100%
rename from semester3/exercises/homework2.c
rename to semester3/exercises/2-Algorighm-Analysis/homework2.c
diff --git a/semester3/exercises/exercise3.10.c b/semester3/exercises/3-Lists-Stacks-and-Queues/exercise3.10.c
similarity index 100%
rename from semester3/exercises/exercise3.10.c
rename to semester3/exercises/3-Lists-Stacks-and-Queues/exercise3.10.c
diff --git a/semester3/exercises/exercise3.21.c b/semester3/exercises/3-Lists-Stacks-and-Queues/exercise3.21.c
similarity index 100%
rename from semester3/exercises/exercise3.21.c
rename to semester3/exercises/3-Lists-Stacks-and-Queues/exercise3.21.c
diff --git a/semester3/exercises/3-Lists-Stacks-and-Queues/exercise3.26.c b/semester3/exercises/3-Lists-Stacks-and-Queues/exercise3.26.c
new file mode 100644
index 0000000..194a310
--- /dev/null
+++ b/semester3/exercises/3-Lists-Stacks-and-Queues/exercise3.26.c
@@ -0,0 +1,100 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef struct deque {
+  int capacity;
+  int front;
+  int rear;
+  int* arr;
+} deque;
+
+deque* new_deque(int capacity) {
+  // capacity > 1
+  deque* res = malloc(sizeof(deque));
+  int* arrray = malloc(sizeof(int) * capacity);
+
+  res->arr = arrray;
+  res->rear = 0;
+  res->front = 1;
+  res->capacity = capacity;
+
+  return res;
+}
+
+void remove_deque(deque *de) {
+  free(de->arr);
+  free(de);
+}
+
+int front_it (deque *de, int step) {
+  int after = (de->front + step + de->capacity) % de->capacity;
+  // check for after
+  if (after == de->rear) {
+    if (step == 1) {
+      printf("ERROR: capacity overload\n");
+      exit(1);
+    } else {
+      printf("ERROR: The array is already empty\n");
+      exit(2);
+    }
+  }
+  de->front = after;
+  return after;
+}
+
+void push (int x, deque* de) {
+  // insert item x to the front end of deque
+  de->arr[front_it(de, 1)] = x;
+}
+
+int pop (deque* de) {
+  // remove the front item and return it.
+  int val = de->arr[de->front];
+  front_it(de, -1);
+  return val;
+}
+
+int rear_it (deque *de, int step) {
+  int after = (de->rear + step + de->capacity) % de->capacity;
+  // check for after
+  if (after == de->front) {
+    if (step == -1) {
+      printf("ERROR: capacity overload\n");
+      exit(1);
+    } else {
+      printf("ERROR: The array is already empty\n");
+      exit(2);
+    }
+  }
+  de->rear = after;
+  return after;
+}
+
+void inject (int x, deque* de) {
+  // rear end
+  de->arr[rear_it(de, -1)] = x;
+}
+
+int eject (deque *de) {
+  // rear end
+  int val = de->arr[de->rear];
+  rear_it(de, 1);
+  return val;
+}
+
+int main(void) {
+  deque* test = new_deque(5);
+  push(1, test);
+  push(2, test);
+  push(3, test);
+  printf("%d\n", pop(test));
+  printf("%d\n", pop(test));
+  printf("%d\n", pop(test));
+  inject(4, test);
+  inject(5, test);
+  printf("REAR: %d\n", eject(test));
+  printf("REAR: %d\n", eject(test));
+
+  remove_deque(test);
+  return 0;
+}
diff --git a/semester3/exercises/exercise3.6.c b/semester3/exercises/3-Lists-Stacks-and-Queues/exercise3.6.c
similarity index 100%
rename from semester3/exercises/exercise3.6.c
rename to semester3/exercises/3-Lists-Stacks-and-Queues/exercise3.6.c
diff --git a/semester3/exercises/4-Trees/exercise4.35.cpp b/semester3/exercises/4-Trees/exercise4.35.cpp
new file mode 100644
index 0000000..ce30043
--- /dev/null
+++ b/semester3/exercises/4-Trees/exercise4.35.cpp
@@ -0,0 +1,38 @@
+#include <queue>
+#include <cstdio>
+
+typedef struct TreeNode {
+  struct TreeNode *l;
+  struct TreeNode *r;
+  int val;
+} TreeNode;
+
+// answer begins here
+void BFS(TreeNode *node) {
+  std::queue<TreeNode *> todo;
+  if (node) {
+    todo.push(node);
+  }
+
+  while (!todo.empty()) {
+    for (int i = 0, size = todo.size(); i < size; i++) {
+      auto ptr = todo.front();
+      std::printf("%d ", ptr->val);
+      todo.pop();
+
+      if (ptr->l) {
+        todo.push(ptr->l);
+      }
+      if (ptr->r) {
+        todo.push(ptr->r);
+      }
+    }
+    printf("\n");
+  }
+}
+
+// answer ends
+int main(void) {
+  BFS(nullptr);
+  return 0;
+}