Merge pull request #24 from Shinh18/issue-8

Add Sentinel linked list 
Thanks @Shinh18

LINKED_LIST/SentinelLinkedList.cpp

@@ -0,0 +1,87 @@
+#include <iostream>
+
+template <typename T>
+class Sentinel {
+    struct Node {
+        T data_;
+        Node* next_;
+        Node* prev_;
+        Node(const T& data=T{}, Node* next = nullptr, Node* prev = nullptr) {
+            data_ = data;
+            next_ = next;
+            prev_ = prev;
+        }
+    };
+    Node* front_;
+    Node* back_;
+public:
+    Sentinel() {
+        front_ = new Node();
+        back_ = new Node();
+        front_->next_ = back_;
+        back_->prev_ = front_;
+    }
+    void insert_front(const T& data);
+    void insert_back(const T& data);
+    void delete_front();
+    void delete_back();
+    void print() const;
+    ~Sentinel();
+};
+
+template <typename T>
+void Sentinel<T>::insert_front(const T& data) {
+    Node* newNode = new Node(data, front_->next_, front_);
+    front_->next_->prev_ = newNode;
+    front_->next_ = newNode;
+}
+
+template <typename T>
+void Sentinel<T>::insert_back(const T& data) {
+    Node* newNode = new Node(data, back_, back_->prev_);
+    back_->prev_->next_ = newNode;
+    back_->prev_ = newNode;
+}
+
+template <typename T>
+void Sentinel<T>::delete_front() {
+    if(front_->next != back_) {
+        Node* rm = front_->next_;
+        front_->next_ = rm->next_;
+        rm->next_->prev_ = front_;
+        delete rm;
+    }
+}
+
+template <typename T>
+void Sentinel<T>::delete_back() {
+    if(front_->next_ != back_) {
+        Node* rm = back_->prev_;
+        back_->prev_ = rm->prev_;
+        back_->prev_->next_ = back_;
+        delete rm;
+    }
+}
+
+template <typename T>
+void Sentinel<T>::print() const {
+    Node* curr = front_->next_;
+    while(curr != back_) {
+        std::cout << curr->data_ << " ";
+        curr = curr->next_;
+    }
+    if(front_->next_ == back_) {
+        std::cout << "empty list";
+    }
+    std::cout << std::endl;
+}
+
+template <typename T>
+Sentinel<T>::~Sentinel() {
+    Node* curr = front_->next_;
+    while(curr != back_) {
+        Node* rm = curr;
+        curr = curr->next_;
+        delete rm;
+    }
+}