diff --git a/A4.cpp b/A4.cpp
new file mode 100644
index 0000000..930274a
--- /dev/null
+++ b/A4.cpp
@@ -0,0 +1,222 @@
+#include <iostream>
+#include <string>
+
+using std::cout;
+using std::cin;
+
+class AVL_tree {
+ public:
+  explicit AVL_tree(int k) : key(k), left(nullptr), right(nullptr), height(1) {}
+  AVL_tree *insert(AVL_tree *p, int k);
+  bool findkey(AVL_tree *p, int key);
+  static std::string next(AVL_tree *p, int k);
+  std::string prev(AVL_tree *p, int k);
+  AVL_tree *remove(AVL_tree *p, int k);
+  ~AVL_tree();
+ private:
+  static int true_height(AVL_tree *p);
+  static int bifactor(AVL_tree *p);
+  void fixheight(AVL_tree *p);
+  AVL_tree *rotateright(AVL_tree *p);
+  AVL_tree *rotateleft(AVL_tree *q);
+  AVL_tree *balance(AVL_tree *p);
+  AVL_tree *findmin(AVL_tree *p);
+  AVL_tree *removemin(AVL_tree *p);
+  int height;
+  AVL_tree *left;
+  AVL_tree *right;
+  int key;
+};
+
+int AVL_tree::true_height(AVL_tree *p) {
+  if (p != nullptr) {
+    return p->height;
+  }
+  return 0;
+}
+
+int AVL_tree::bifactor(AVL_tree *p) {
+  return AVL_tree::true_height(p->right) - AVL_tree::true_height(p->left);
+}
+
+void AVL_tree::fixheight(AVL_tree *p) {
+  int hl = true_height(p->left);
+  int hr = true_height(p->right);
+  p->height = std::max(hl, hr) + 1;
+}
+
+AVL_tree *AVL_tree::rotateright(AVL_tree *p) {
+  AVL_tree *q = p->left;
+  p->left = q->right;
+  q->right = p;
+  fixheight(p);
+  fixheight(q);
+  return q;
+}
+
+AVL_tree *AVL_tree::rotateleft(AVL_tree *q) {
+  AVL_tree *p = q->right;
+  q->right = p->left;
+  p->left = q;
+  fixheight(q);
+  fixheight(p);
+  return p;
+}
+
+AVL_tree *AVL_tree::balance(AVL_tree *p) {
+  fixheight(p);
+  if (bifactor(p) == 2) {
+    if (bifactor(p->right) < 0) {
+      p->right = rotateright(p->right);
+    }
+    return rotateleft(p);
+  }
+  if (bifactor(p) == -2) {
+    if (bifactor(p->left) > 0) {
+      p->left = rotateleft(p->left);
+    }
+    return rotateright(p);
+  }
+  return p;
+}
+
+AVL_tree *AVL_tree::insert(AVL_tree *p, int k) {
+  if (p == nullptr) return new AVL_tree(k);
+  if (k < p->key)
+    p->left = insert(p->left, k);
+  else
+    p->right = insert(p->right, k);
+  return balance(p);
+}
+
+AVL_tree *AVL_tree::findmin(AVL_tree *p) {
+  if (p->left) {
+    return findmin(p->left);
+  } else return p;
+}
+
+bool AVL_tree::findkey(AVL_tree *p, int key) {
+  if (p == nullptr) {
+    return false;
+  }
+  if (key < p->key) {
+    findkey(p->left, key);
+  } else {
+    if (key == p->key) {
+      return true;
+    } else {
+      findkey(p->right, key);
+    }
+  }
+}
+
+AVL_tree *AVL_tree::removemin(AVL_tree *p) {
+  if (p->left == nullptr) {
+    return p->right;
+  }
+  p->left = removemin(p->left);
+  return balance(p);
+}
+
+AVL_tree *AVL_tree::remove(AVL_tree *p, int k) {
+  if (p == nullptr) {
+    return nullptr;
+  }
+  if (k < p->key) {
+    p->left = remove(p->left, k);
+  } else {
+    if (k > p->key) {
+      p->right = remove(p->right, k);
+    } else {
+      AVL_tree *q = p->left;
+      AVL_tree *r = p->right;
+      delete p;
+      if (r == nullptr) return q;
+      AVL_tree *min = findmin(r);
+      min->right = removemin(r);
+      min->left = q;
+      return balance(min);
+    }
+  }
+  return balance(p);
+}
+
+std::string AVL_tree::next(AVL_tree *p, int k) {
+  if (p == nullptr) {
+    return "none";
+  }
+  AVL_tree *pip = p;
+  AVL_tree *otv = nullptr;
+  while (pip != nullptr) {
+    if (pip->key > k) {
+      otv = pip;
+      pip = pip->left;
+    } else {
+      pip = pip->right;
+    }
+  }
+  if (otv == nullptr) {
+    return "none";
+  } else {
+    return std::to_string(otv->key);
+  }
+}
+
+std::string AVL_tree::prev(AVL_tree *p, int k) {
+  if (p == nullptr) {
+    return "none";
+  }
+  AVL_tree *pip = p;
+  AVL_tree *otv = nullptr;
+  while (pip != nullptr) {
+    if (pip->key >= k) {
+      pip = pip->left;
+    } else {
+      otv = pip;
+      pip = pip->right;
+    }
+  }
+  if (otv == nullptr) {
+    return "none";
+  } else {
+    return std::to_string(otv->key);
+  }
+}
+AVL_tree::~AVL_tree() {
+  left = nullptr;
+  right = nullptr;
+  height = 0;
+  key = 0;
+}
+
+int main() {
+  AVL_tree *L = nullptr;
+  std::string s;
+  while (cin >> s) {
+    int k;
+    cin >> k;
+    if (s == "insert") {
+      if (!L->findkey(L, k)) {
+        L = L->insert(L, k);
+      }
+    }
+    if (s == "exists") {
+      if (L->findkey(L, k)) {
+        cout << "true\n";
+      } else {
+        cout << "false\n";
+      }
+    }
+    if (s == "delete") {
+      if (L->findkey(L, k)) {
+        L = L->remove(L, k);
+      }
+    }
+    if (s == "next") {
+      cout << L->next(L, k) << "\n";
+    }
+    if (s == "prev") {
+      cout << L->prev(L, k) << "\n";
+    }
+  }
+}
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