diff --git a/thu_dsa/chp3/List.h b/thu_dsa/chp3/List.h
index 6094a6e..c91452c 100644
--- a/thu_dsa/chp3/List.h
+++ b/thu_dsa/chp3/List.h
@@ -27,10 +27,12 @@ public:
 	//read-only interfaces
 	ListNodePosi(T) first() const { return head->succ; }
 	ListNodePosi(T) last() const { return tail->prev; }
+	ListNodePosi(T) prev(ListNodePosi(T) p) const { return p->prev; }
+	ListNodePosi(T) next(ListNodePosi(T) p) const { return p->next; }
 	ListNodePosi(T) find(T const &val) const { return find(val, size, tail); }		//find val from all elements of the list
 	ListNodePosi(T) find(T const &val, int n, ListNodePosi(T) p) const;				//find val from a range of n elements before p
 	ListNodePosi(T) search(T const &val, int n, ListNodePosi(T) p) const;			//find val in a sorted List<T>
-	ListNodePosi(T) search(T const &val) const { return search(val, size, tail); }	//find val universally ina sorted List<T>
+	ListNodePosi(T) search(T const &val) const { return search(val, size, tail); }	//find val universally in a sorted List<T>
 	T& operator[](int rank);
 	int getSize() const { return size; }
 	bool empty() const { return size == 0; }
diff --git a/thu_dsa/chp9/dict.h b/thu_dsa/chp9/dict.h
new file mode 100644
index 0000000..b5d121d
--- /dev/null
+++ b/thu_dsa/chp9/dict.h
@@ -0,0 +1,14 @@
+#ifndef DICT_H_
+#define DICT_H_
+
+//Dict virtual class
+template <typename K, typename V>
+class Dict{
+public:
+	virtual int  size() = 0;
+	virtual V    get(K key) = 0;
+	virtual bool put(K key, V value) = 0;
+	virtual bool remove(K key) = 0;
+};
+
+#endif
diff --git a/thu_dsa/chp9/quadlist.h b/thu_dsa/chp9/quadlist.h
new file mode 100644
index 0000000..e973eda
--- /dev/null
+++ b/thu_dsa/chp9/quadlist.h
@@ -0,0 +1,88 @@
+#ifndef QUADLIST_H_
+#define QUADLIST_H_
+
+#define QuadNodePosi(T) QuadNode<T>*
+#define MAXINT (int)(0x7fffffff)
+#define MININT (int)(0x80000000)
+
+template <typename T>
+class QuadNode{
+public:
+	T entry;
+	QuadNodePosi(T) prev;
+	QuadNodePosi(T) next;
+	QuadNodePosi(T) above;
+	QuadNodePosi(T) below;
+
+	QuadNode() = default;
+	QuadNode(T entry) :entry(entry) {}
+};
+
+template <typename T>
+class QuadList{
+private:
+	QuadNodePosi(T) __head;
+	QuadNodePosi(T) __tail;
+	int             __size;
+
+public:
+	//constructor
+	QuadList();
+
+	//deconstrutor
+	~QuadList();
+
+	//external interfaces
+
+	//read-only interfaces
+	int  size()  const { return __size; }
+	bool empty() const { return size() == 0; }
+	QuadNodePosi(T) first() const { return empty()? nullptr : __head->next;}
+	QuadNodePosi(T) last()  const { return empty()? nullptr : __tail->prev;}
+	QuadNodePosi(T) prev(QuadNodePosi(T) p)  const { return p->prev; }
+	QuadNodePosi(T) next(QuadNodePosi(T) p)  const { return p->next; }
+
+	//writable interfaces
+	void insert_before_above(QuadNodePosi(T) p, T const& entry, QuadNodePosi(T) base = nullptr);
+	void insert_after_above(QuadNodePosi(T)p, T const & entry, QuadNodePosi(T) base = nullptr);
+	void remove(QuadNodePosi(T) p);
+};
+
+//constructor
+template <typename T>
+QuadList<T>::QuadList(){
+	__head = new QuadNode<T>(MININT);
+	__tail = new QuadNode<T>(MAXINT);
+	__size = 0;
+	__head->next = __tail;
+	__tail->prev = __head;
+}
+
+//writable interfaces
+
+template <typename T>
+void QuadList<T>::insert_before_above(QuadNodePosi(T) p, T const & entry, QuadNodePosi(T) base){
+	QuadNodePosi(T) newNode = new QuadNode<T>(entry);
+	newNode->next = p;
+	newNode->prev = p->prev;
+	p->prev = newNode;
+	newNode->prev->next = newNode;
+	base->above = newNode;
+	newNode->below = base;
+	++__size;
+}
+
+template <typename T>
+void QuadList<T>::insert_after_above(QuadNodePosi(T) p, T const & entry, QuadNodePosi(T) base) {
+	insert_before_above(p->prev, entry, base);
+}
+
+template <typename T>
+void QuadList<T>::remove(QuadNodePosi(T) p){
+	//assert(p != head && p != tail);
+	p->prev->next = p->next;
+	p->next->prev = p->prev;
+	--__size;
+}
+
+#endif
diff --git a/thu_dsa/chp9/skiplist.h b/thu_dsa/chp9/skiplist.h
new file mode 100644
index 0000000..547dac9
--- /dev/null
+++ b/thu_dsa/chp9/skiplist.h
@@ -0,0 +1,123 @@
+#ifndef SKIPLIST_H_
+#define SKIPLIST_H_
+
+#include "dict.h"
+#include "quadlist.h"
+#include "../chp3/List.h"
+#include <iostream>
+#include <cstdlib>
+#include <ctime>
+
+using std::cerr;
+using std::endl;
+
+template <typename K, typename V>
+class entry{
+public:
+	K key;
+	V value;
+
+	entry() = default;
+	entry(K key, V value) : key(key), value(value) {}
+
+	operator==(entry const &e){
+		return key == e.key;
+	}
+};
+
+template <typename K, typename V>
+class SkipList: Dict<K, V>, List<QuadList<entry<K, V>>*>{
+protected:
+	QuadNode<entry<K, V>>* skipSearch(K key);
+
+public:
+	//constructor
+	SkipList();
+
+	int  size()  const { return first()->size(); }
+	int  level() const { return getSize(); }
+
+	V    get(K key);
+	bool put(K key, V value);
+	bool remove(K key);
+};
+
+//constructor
+template <typename K, typename V>
+SkipList<K, V>::SkipList(){
+	QuadList<entry<K, V>>* qlist = new QuadList<entry<K, V>>();
+	push_back(qlist);
+}
+
+//internal methods
+
+template <typename K, typename V>
+QuadNode<entry<K, V>>* SkipList<K, V>::skipSearch(K key) {
+	while (1) {
+		QuadNode<entry<K, V>>* qnode = last()->first();					//the first quadlist node of the last quadlist
+		while (qnode->entry.key < key) qnode = qnode->next;
+		if (qnode->entry.key == key) return qnode;
+		//else
+		qnode = qnode->prev;
+		if (qnode->below == nullptr) return qnode;
+		//else
+		qnode = qnode->below;
+	}
+}
+
+//external interfaces
+
+template <typename K, typename V>
+V SkipList<K, V>::get(K key){
+	QuadNode<entry<K, V>>* res;
+	res = skipSearch(key);
+	if(res->entry.key != key){
+		cerr << "key:" << key << "is not in SkipList!" << endl;
+	}
+	return res->entry.value;
+}
+
+template <typename K, typename V>
+bool SkipList<K, V>::put(K key, V value){
+	QuadNode<entry<K, V>>* qnode = skipSearch(key);
+	QuadNode<entry<K, V>>* belowNode;
+	ListNode<QuadList<entry<K, V>>*>* qlist = first();
+
+	while (qnode->below != nullptr) qnode = qnode->below;
+	if (qnode->entry.key != key) qnode = qnode->next;
+	qlist->val->insert_before_above(qnode, entry<K, V>(key, value));
+
+	srand((int)time(0));
+	while(rand() & 1){
+		belowNode = qnode->prev;
+		qlist = next(qlist);
+		if(qlist->val->empty()){
+			push_back(new QuadList<entry<K, V>>());
+		}
+		while (qnode->above == nullptr) qnode = qnode->next;
+		qnode = qnode->above;
+		qlist->val->insert_before_above(qnode, entry<K, V>(key, value), belowNode);
+	}
+	return true;
+}
+
+template <typename K, typename V>
+bool SkipList<K, V>::remove(K key){
+	QuadNode<entry<K, V>>* qnode = skipSearch(key);
+	QuadNode<entry<K, V>>* tempNode;
+	ListNode<QuadList<entry<K, V>>*>* qlist = first();
+	if (qnode->entry.key != key) return false;
+	//else
+	while (qnode->below) qnode = qnode->below;
+	
+	while(qnode){
+		tempNode = qnode;
+		qlist->val->remove(qnode);
+		qnode = qnode->above;
+		qlist = qlist->val->next();
+		delete tempNode;
+	}
+	return true;
+}
+
+#endif