diff --git a/2.c b/2.c
new file mode 100644
index 0000000..67c49c2
--- /dev/null
+++ b/2.c
@@ -0,0 +1,93 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef struct Student student;
+struct Student {
+  int sid;
+  struct Student *next;
+};
+
+void insert(student **head, int idx, uint sid) {
+  student *new = malloc(sizeof(student));
+  new->sid = sid;
+
+  int i = 0;
+
+  student *prev = NULL, *cur = *head;
+
+  for (int i = 0;; i++) {
+    if (i == idx) {
+      if (prev != NULL)
+        prev->next = new;
+      else
+        *head = new;
+      new->next = cur;
+      return;
+    }
+
+    if (cur == NULL)
+      return;
+
+    prev = cur;
+    cur = cur->next;
+  }
+
+  printf("Index %d does not exist\n", idx);
+  return;
+}
+
+void delete (student **head, int idx) {
+  student *prev = NULL, *cur = *head;
+
+  for (int i = 0; cur != NULL; i++) {
+    if (i == idx) {
+      if (prev != NULL)
+        prev->next = cur->next;
+      else
+        *head = cur->next;
+      free(cur);
+      return;
+    }
+    prev = cur;
+    cur = cur->next;
+  }
+
+  printf("Index %d does not exist\n", idx);
+  return;
+}
+
+void reverse(student **head) {
+  student *prev = NULL, *cur = *head;
+
+  while (cur->next != NULL) {
+    student *next = cur->next;
+    cur->next = prev;
+    prev = cur;
+    cur = next;
+  }
+
+  cur->next = prev;
+
+  *head = cur;
+  return;
+}
+
+int main() {
+  student *head = NULL;
+  insert(&head, 0, 1);
+  insert(&head, 1, 2002);
+  insert(&head, 2, -9);
+  insert(&head, 3, 52);
+  insert(&head, 1, 4);
+  delete (&head, 2);
+  reverse(&head);
+
+  while (head != NULL) {
+    printf("%d ", head->sid);
+    head = head->next;
+  }
+
+  // free
+  while (head != NULL)
+    delete(&head, 0);
+}
diff --git a/4.c b/4.c
new file mode 100644
index 0000000..7384b30
--- /dev/null
+++ b/4.c
@@ -0,0 +1,70 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef struct Node node;
+struct Node {
+  int collegeid;
+  struct Node *next;
+};
+
+void insert(node **last, int collegeid) {
+  node *dst = malloc(sizeof(node));
+  dst->collegeid = collegeid;
+
+  if (*last == NULL)
+    *last = dst;
+
+  if ((*last)->next == NULL)
+    (*last)->next = dst, dst->next = *last;
+  else {
+    dst->next = (*last)->next;
+    (*last)->next = dst;
+  }
+}
+
+void delete (node **last) {
+  node *prev = NULL;
+
+  if ((*last)->next == NULL)
+    goto cleanup;
+
+  prev = *last;
+
+  while (prev->next != *last)
+    prev = prev->next;
+
+  if (prev->next->next == prev)
+    prev->next = NULL;
+  else
+    prev->next = (*last)->next;
+
+cleanup:
+  free(*last);
+  *last = prev;
+}
+
+void display(node *last) {
+  node *cur = last;
+  while (cur->next != last) {
+    printf("%d ", cur->next->collegeid);
+    cur = cur->next;
+  }
+  printf("%d\n", last->collegeid);
+}
+
+int main() {
+  node *last = NULL;
+
+  insert(&last, 1);
+  insert(&last, 4);
+  insert(&last, 99);
+  insert(&last, 1223);
+  display(last);
+
+  delete (&last);
+  display(last);
+
+  // free
+  while (last != NULL)
+    delete (&last);
+}