diff --git a/OOP/12/Excercise01.cpp b/OOP/12/Excercise01.cpp
new file mode 100644
index 0000000..6e0fe05
--- /dev/null
+++ b/OOP/12/Excercise01.cpp
@@ -0,0 +1,58 @@
+#include <iostream>
+using std::cout;
+using std::endl;
+
+template <class T>
+void swap(T list[], const int left, const int right) {
+    T tmp = list[left];
+    list[left] = list[right];
+    list[right] = tmp;
+}
+
+template <class T>
+T *sort(T originList[], const int left, const int right) {
+    int l = left, r = right;
+    int pivot = left;
+    if (left >= right) {
+        return originList;
+    }
+    while (l < r) {
+        while (originList[r] >= originList[pivot] && r > l) {
+            r--;
+        }
+        swap(originList, pivot, r);
+        pivot = r;
+        while (originList[l] <= originList[pivot] && l < r) {
+            l++;
+        }
+        swap(originList, pivot, l);
+        pivot = l;
+    }
+    sort(originList, left, pivot - 1);
+    sort(originList, pivot + 1, right);
+    return originList;
+}
+
+int main() {
+    int intList[6] = {1, 6, 4, 5, 2, 3};
+    char charList[14] = "Muelsyse.com!";
+    for (int i = 0; i < 6; i++) {
+        cout << intList[i] << ' ';
+    }
+    cout << endl;
+    for (int i = 0; i < 13; i++) {
+        cout << charList[i] << ' ';
+    }
+    cout << endl;
+    sort(intList, 0, 5);
+    sort(charList, 0, 9);
+    for (int i = 0; i < 6; i++) {
+        cout << intList[i] << ' ';
+    }
+    cout << endl;
+    for (int i = 0; i < 13; i++) {
+        cout << charList[i] << ' ';
+    }
+    cout << endl;
+    return 0;
+}
\ No newline at end of file
diff --git a/OOP/12/Exercise02.cpp b/OOP/12/Exercise02.cpp
new file mode 100644
index 0000000..f60cf55
--- /dev/null
+++ b/OOP/12/Exercise02.cpp
@@ -0,0 +1,96 @@
+#include <iostream>
+using std::cout;
+using std::endl;
+
+template <class E> class Storage {
+private:
+    E *objectEptr;
+    int length;
+    void swap(int left, int right) {
+        E tmp = objectEptr[left];
+        objectEptr[left] = objectEptr[right];
+        objectEptr[right] = tmp;
+    }
+    void quickSort(const int left, const int right) {
+        if (left >= right) {
+            return;
+        }
+        int l = left, r = right;
+        int pivot = left;
+        while (l < r) {
+            while (objectEptr[r] >= objectEptr[pivot] && r > l) {
+                r--;
+            }
+            this->swap(pivot, r);
+            pivot = r;
+            while (objectEptr[l] <= objectEptr[pivot] && l < r) {
+                l++;
+            }
+            this->swap(pivot, l);
+            pivot = l;
+        }
+        this->quickSort(left, pivot - 1);
+        this->quickSort(pivot + 1, right);
+    }
+
+public:
+    Storage() : objectEptr(NULL){};
+    Storage(const int 
+    _length) : objectEptr(new E[_length]), length(_length){};
+    Storage(const E *originptr, const int _n)
+        : objectEptr(new E[_n]), length(_n) {
+        for (int i = 0; i < _n; i++) {
+            objectEptr[i] = originptr[i];
+        }
+    };
+    ~Storage() {
+        delete[] objectEptr;
+    }
+    Storage &sort() {
+        quickSort(0, length - 1);
+        return *this;
+    }
+    Storage &sort(const int leftIndex, const int rightIndex) {
+        quickSort(leftIndex, rightIndex);
+        return *this;
+    }
+    E *getList() {
+        return this->objectEptr;
+    }
+    int getLength() {
+        return this->length;
+    }
+    Storage &printOut() {
+        for (int i = 0; i < this->length; i++) {
+            cout << objectEptr[i] << ' ';
+        }
+        cout << endl;
+        return *this;
+    }
+    E &operator[](int index) {
+        return objectEptr[index];
+    }
+
+    int search(E target) {
+        for (int i = 0; i < this->length; i++) {
+            if (target == objectEptr[i]) {
+                return i;
+            }
+        }
+        return -1;
+    }
+};
+
+int main() {
+    int list[6] = {1, 4, 6, 5, 3, 2};
+    std::string str = "HelloWorld!";
+    Storage<int> aList(list, 6);
+    Storage<char> charList(str.c_str(), str.length() + 1);
+    aList.printOut().sort().printOut();
+    charList.printOut().sort(0, charList.getLength() - 2).printOut();
+    aList[2] = 8;
+    aList.printOut();
+    cout << "8 is at index: " << aList.search(8) << endl;
+    cout << "H is at index: " << charList.search('H') << endl;
+    return 0;
+}
\ No newline at end of file
diff --git a/OOP/12/Optional01/ListExceptions.h b/OOP/12/Optional01/ListExceptions.h
new file mode 100644
index 0000000..dc9ed17
--- /dev/null
+++ b/OOP/12/Optional01/ListExceptions.h
@@ -0,0 +1,8 @@
+#pragma once
+class BaseListExceptions {};
+
+class ValueError : public BaseListExceptions {};
+
+class IndexError : public BaseListExceptions {};
+
+class DuplicateError : public BaseListExceptions {};
\ No newline at end of file
diff --git a/OOP/12/Optional01/listE.h b/OOP/12/Optional01/listE.h
new file mode 100755
index 0000000..70b2f5c
--- /dev/null
+++ b/OOP/12/Optional01/listE.h
@@ -0,0 +1,163 @@
+#pragma once
+#include "ListExceptions.h"
+#include "node.h"
+
+template <class E> class List {
+private:
+    Node<E> *head;
+    int _length;
+    int getIndexInRange(int index);
+
+public:
+    List();
+    List(const E &newItem);
+    List(const E *newItemList, const int itemCount);
+    ~List();
+    // Append _newItem at last. Returns the List itself.
+    List<E> &append(const E &_newItem);
+    // Insert _newItem at index, and move all items at and after [index] 1 step.
+    // Throws IndexError if index is not in the range of [-_length, _length -
+    // 1]. Returns the List itself.
+    List<E> &insert(int index, const E &_newItem);
+    // Pop the item at given index, default is the to pop the last one.
+    E pop(int index = -1);
+    // Remove the first occurance of target. Throws ValueError if target does
+    // not exist. Returns the List itself.
+    List<E> &remove(const E &target);
+    // Clear all nodes.
+    List<E> &clear();
+    E &operator[](const int index);
+
+    int length();
+    bool contains(const E &target);
+    // Give the index of target in the list on its first appearance. Throws
+    // ValueError if target does not exist.
+    int index(const E &target);
+};
+
+template <class E> int List<E>::getIndexInRange(int index) {
+    if (index < 0) {
+        index += this->_length;
+    }
+    if (index < 0 || index >= _length) {
+        throw(IndexError());
+    }
+    return index;
+}
+
+template <class E> List<E>::List() : head(NULL), _length(0){};
+
+template <class E>
+List<E>::List(const E &newItem) : head(new Node<E>(newItem)), _length(1){};
+
+template <class E> List<E>::List(const E *newItemList, const int itemCount) {
+    for (int i = 0; i < itemCount; i++) {
+        this->append(newItemList[i]);
+    }
+};
+
+template <class E> List<E>::~List() {
+    if (this->head != NULL) {
+        this->head->destruct();
+    }
+    head = NULL;
+}
+
+template <class E> List<E> &List<E>::append(const E &_newItem) {
+    if (this->head == NULL) {
+        this->head = new Node<E>(_newItem);
+    }
+    else {
+        this->head->getTail().setNext(new Node<E>(_newItem));
+    }
+    this->_length++;
+    return *this;
+}
+
+template <class E> List<E> &List<E>::insert(int index, const E &_newItem) {
+    index = this->getIndexInRange(index);
+    if (index == 0) {
+        this->head = new Node<E>(_newItem, this->head);
+    }
+    else {
+        Node<E> &prev = this->head->getByIndex(index - 1);
+        prev.setNext(new Node<E>(_newItem, prev.getNextPtr()));
+    }
+    this->_length++;
+    return *this;
+}
+
+template <class E> List<E> &List<E>::remove(const E &target) {
+    this->pop(this->index(target));
+    return *this;
+}
+
+template <class E> E List<E>::pop(int index) {
+    if (_length == 0) {
+        throw(IndexError());
+    }
+
+    index = this->getIndexInRange(index);
+
+    Node<E> removed = this->head->getByIndex(index);
+
+    this->_length--;
+    if (index == 0) {
+        Node<E> *newHead = this->head->getNextPtr();
+        delete this->head;
+        this->head = newHead;
+    }
+    else {
+        Node<E> &prev = this->head->getByIndex(index - 1);
+        delete prev.getNextPtr();
+        prev.setNext(removed.getNextPtr());
+    }
+    return removed.getContent();
+}
+
+template <class E> List<E> &List<E>::clear() {
+    this->head->destruct();
+    this->head = NULL;
+    this->_length = 0;
+    return *this;
+}
+
+template <class E> E &List<E>::operator[](const int index) {
+    return this->head->getByIndex(getIndexInRange(index)).getContent();
+}
+
+template <class E> int List<E>::length() {
+    return this->_length;
+}
+
+template <class E> bool List<E>::contains(const E &target) {
+    if (this->_length == 0) {
+        return false;
+    }
+
+    Node<E> *current = this->head;
+    do {
+        if (current->getContent() == target) {
+            return true;
+        }
+        current = current->getNextPtr();
+    } while (current != NULL);
+    return false;
+}
+
+template <class E> int List<E>::index(const E &target) {
+    if (this->_length == 0) {
+        throw(ValueError());
+    }
+
+    Node<E> *current = this->head;
+    int pos = 0;
+    do {
+        if (current->getContent() == target) {
+            return pos;
+        }
+        current = current->getNextPtr();
+        pos++;
+    } while (current != NULL);
+    throw(ValueError());
+}
\ No newline at end of file
diff --git a/OOP/12/Optional01/main.cpp b/OOP/12/Optional01/main.cpp
new file mode 100755
index 0000000..e80b671
--- /dev/null
+++ b/OOP/12/Optional01/main.cpp
@@ -0,0 +1,65 @@
+#include "listE.h"
+#include "setE.h"
+#include <iostream>
+using std::cout;
+using std::endl;
+
+template <class E> void output(List<E>& list) {
+    for (int i = 0; i < list.length(); i++) {
+        cout << list[i] << ' ';
+    }
+    if (list.length() == 0) {
+        cout << "(Empty)";
+    }
+    cout << endl;
+}
+
+int main() {
+    int nums[5] = {1, 4, 6, 8, 3};
+    List<int> aList;
+    List<int> bList(nums, 5);
+    output(bList);
+    try {
+        aList.remove(0);
+    }
+    catch(ValueError) {
+        std::cerr << "There is an ValueError" << std::endl;
+    }
+    aList.append(3);
+    aList.append(8);
+    aList.append(9);
+    aList.append(12);
+    aList.insert(1, 20);
+    output(aList);
+
+    cout << aList.pop() << endl;
+    output(aList);
+
+    aList.pop(2);
+    output(aList);
+
+    cout << aList.contains(3) << ' ' << aList.contains(15) << endl;
+
+    try {
+        aList.index(15);
+    }
+    catch(ValueError) {
+        std::cerr << "There's no 15 in this list." << std::endl;
+    }
+
+    aList.clear();
+    output(aList);
+
+    cout << "\n----------\n" << endl;
+
+    Set<int> aSet;
+    aSet.show();
+
+    aSet.add(2);
+    aSet.add(3);
+    aSet.show();
+
+    aSet.remove(2);
+    aSet.show();
+    return 0;
+}
\ No newline at end of file
diff --git a/OOP/12/Optional01/node.h b/OOP/12/Optional01/node.h
new file mode 100755
index 0000000..bdcf4a6
--- /dev/null
+++ b/OOP/12/Optional01/node.h
@@ -0,0 +1,67 @@
+#pragma once
+#include <iostream>
+
+template <class E> class Node {
+private:
+    E content;
+    Node *nextNode;
+
+public:
+    Node();
+    Node(E _content, Node<E> *_nextNode = NULL);
+    ~Node();
+    // Returns a reference to the content.
+    E &getContent();
+    // Returns a reference to the next Node. May return NULL if this Node is the
+    // last one.
+    Node<E> *getNextPtr();
+    // Set the next Node _nextNode. Returns THIS Node.
+    Node<E> &setNext(Node<E> *_nextNode);
+    // Returns a reference to the Tail.
+    Node<E> &getTail();
+    // Returns a reference to the Node of given index.
+    Node<E> &getByIndex(const int index);
+    void destruct();
+};
+
+template <class E> Node<E>::Node() : content(0), nextNode(NULL){};
+
+template <class E>
+Node<E>::Node(E _content, Node *_nextNode)
+    : content(_content), nextNode(_nextNode){};
+
+template <class E> Node<E>::~Node(){};
+
+template <class E> E &Node<E>::getContent() {
+    return this->content;
+}
+
+template <class E> Node<E> *Node<E>::getNextPtr() {
+    return this->nextNode;
+}
+
+template <class E> Node<E> &Node<E>::setNext(Node<E> *_nextNode) {
+    this->nextNode = _nextNode;
+    return *this;
+}
+
+template <class E> Node<E> &Node<E>::getTail() {
+    if (this->nextNode == NULL) {
+        return *this;
+    }
+    return this->nextNode->getTail();
+}
+
+template <class E> Node<E> &Node<E>::getByIndex(const int index) {
+    if (index == 0) {
+        return *this;
+    }
+    return this->nextNode->getByIndex(index - 1);
+}
+
+template <class E> void Node<E>::destruct() {
+    if (this->nextNode != NULL) {
+        this->nextNode->destruct();
+    }
+    delete this;
+}
\ No newline at end of file
diff --git a/OOP/12/Optional01/setE.h b/OOP/12/Optional01/setE.h
new file mode 100644
index 0000000..bc49859
--- /dev/null
+++ b/OOP/12/Optional01/setE.h
@@ -0,0 +1,69 @@
+#include "ListExceptions.h"
+#include "listE.h"
+
+template <class E> class Set {
+private:
+    List<E> _list;
+
+public:
+    Set();
+    Set(const E &_newItem);
+    Set(const E *_newItem, const int count);
+    // Add an element.
+    Set<E> &add(const E &_newItem);
+    // Remove an element.
+    Set<E> &remove(const E &target);
+    // Clear all elements.
+    Set<E> &clear();
+
+    int size();
+    void show();
+};
+
+template <class E> Set<E>::Set() : _list(){};
+
+template <class E> Set<E>::Set(const E &_newItem) : _list(_newItem){};
+
+template <class E> Set<E>::Set(const E *_newItem, const int count) : _list() {
+    for (int i = 0; i < count; i++) {
+        try {
+            this->add(_newItem[i]);
+        }
+        catch (DuplicateError) {
+            continue;
+        }
+    }
+};
+
+template <class E> Set<E> &Set<E>::add(const E &_newItem) {
+    if (_list.contains(_newItem)) {
+        throw(DuplicateError());
+    }
+
+    _list.append(_newItem);
+    return *this;
+};
+
+template <class E> Set<E> &Set<E>::remove(const E &target) {
+    _list.remove(target);
+    return *this;
+}
+
+template <class E> Set<E> &Set<E>::clear() {
+    _list.clear();
+    return *this;
+}
+
+template <class E> int Set<E>::size() {
+    return _list.length();
+}
+
+template <class E> void Set<E>::show() {
+    for (int i = 0; i < _list.length(); i++) {
+        std::cout << _list[i] << ' ';
+    }
+    if (_list.length() == 0) {
+        std::cout << "(Empty)";
+    }
+    std::cout << std::endl;
+}
\ No newline at end of file
diff --git a/OOP/test.cpp b/OOP/test.cpp
index 975311e..d3a898f 100644
--- a/OOP/test.cpp
+++ b/OOP/test.cpp
@@ -59,6 +59,12 @@ public:
 int main() {
     // D objD(1, 2, 3, 4);
     // objD.display();
-    cout << "---\n" << sizeof(A) << endl << sizeof(B) << endl << sizeof(C) << endl;
+    // cout << "---\n" << sizeof(A) << endl << sizeof(B) << endl << sizeof(C) << endl;
+    // cout << -13 % 6 << endl;
+    cout << "---";
+    endl(cout);
+    endl(cout);
+    cout << "---";
+    endl(cout);
     return 0;
 }
\ No newline at end of file
diff --git a/POP/13/Optional01.c b/POP/13/Optional01.c
index cc1f8cc..8057360 100644
--- a/POP/13/Optional01.c
+++ b/POP/13/Optional01.c
@@ -60,7 +60,7 @@ NODE* insert(NODE* head, int index, int value) {
     NODE* newNodePtr;
     currentNode = head;
     if ((*currentNode).index > index) {
-        newNodePtr = malloc(LEN);
+        newNodePtr = (NODE*) malloc(LEN);
         (*newNodePtr).index = index;
         (*newNodePtr).value = value;
         (*newNodePtr).prev = NULL;
@@ -77,7 +77,7 @@ NODE* insert(NODE* head, int index, int value) {
             currentNode = (*currentNode).next;
             continue;
         }
-        newNodePtr = malloc(LEN);
+        newNodePtr = (NODE*)malloc(LEN);
         (*newNodePtr).index = index;
         (*newNodePtr).value = value;
         (*newNodePtr).prev = currentNode;
@@ -86,7 +86,7 @@ NODE* insert(NODE* head, int index, int value) {
         (*currentNode).next = newNodePtr;
         return newNodePtr;
     }
-    newNodePtr = malloc(LEN);
+    newNodePtr = (NODE*)malloc(LEN);
     (*newNodePtr).index = index;
     (*newNodePtr).value = value;
     (*newNodePtr).prev = currentNode;
@@ -97,7 +97,7 @@ NODE* insert(NODE* head, int index, int value) {
 
 NODE* create(int index, int value) {
     NODE* newNodePtr;
-    newNodePtr = malloc(LEN);
+    newNodePtr = (NODE*)malloc(LEN);
     (*newNodePtr).index = index;
     (*newNodePtr).value = value;
     (*newNodePtr).prev = NULL;