adding leetcode doubly linked list problem

lectures/11_list_implementation/README.md

@@ -83,4 +83,4 @@ the first or last node.
 ## 11.8 Leetcode Exercises
 
 - [Leetcode problem 141: Linked List Cycle](https://leetcode.com/problems/linked-list-cycle/). Solution: [p141_linkedlistcycle.cpp](../../leetcode/p141_linkedlistcycle.cpp)
-
+- [Leetcode problem 1472: Design Browser History](https://leetcode.com/problems/design-browser-history/). Solution: [p1472_browserhistory.cpp](../../leetcode/p1472_browserhistory.cpp)

leetcode/p1472_browserhistory.cpp

@@ -0,0 +1,107 @@
+class Node{
+public:
+    // default constructor
+    Node(){
+        url="";
+        next=nullptr;
+        prev=nullptr;
+    }
+    // another constructor
+    Node(string s){
+        url=s;
+        next=nullptr;
+        prev=nullptr;
+    }
+    std::string getURL(){
+        return url;
+    }
+    Node* getNext(){
+        return next;
+    }
+    Node* getPrev(){
+        return prev;
+    }
+    void setNext(Node* p){
+        next = p;
+    }
+    void setPrev(Node* p){
+        prev = p;
+    }
+private:
+    Node *next;
+    Node *prev;
+    std::string url;
+};
+
+void PushBack(Node* &head, Node* &tail, Node* &node){
+    // edge case, when there is nothing in the linked list
+    if(tail==nullptr){
+        head = node;
+        tail = node;
+        return;
+    }
+    tail->setNext(node);
+    node->setPrev(tail);
+    tail = node;
+}
+
+void PopBack(Node* &head, Node* &tail){
+    Node* temp = tail;
+    tail = tail->getPrev();
+    tail->setNext(nullptr);
+    // delete the original tail
+    delete temp;
+}
+
+class BrowserHistory {
+public:
+    BrowserHistory(string homepage) {
+        current = new Node(homepage);
+        head = tail = current;
+    }
+    
+    void visit(string url) {
+        Node* temp = new Node(url);
+        // if current is not equal to tail, delete whatever is in between current and tail (including tail); because the requirement is "clears up all the forward history."
+        while(current!=tail){
+            PopBack(head, tail);
+        }
+        PushBack(head, tail, temp);
+        current = tail;
+    }
+    
+    string back(int steps) {
+        if(head==nullptr){
+            return "";
+        }
+        // go back "steps" steps
+        for(int i=0;i<steps&&current!=head;i++){
+            current = current->getPrev();
+        }
+        return current->getURL();
+    }
+    
+    string forward(int steps) {
+        if(head==nullptr){
+            return "";
+        }
+        // go forward "steps" steps
+        for(int i=0;i<steps&&current!=tail;i++){
+            current = current->getNext();
+        }
+        return current->getURL();
+    }
+
+private:
+    Node* head;
+    Node* tail;
+    Node* current;
+};
+
+/**
+ * Your BrowserHistory object will be instantiated and called as such:
+ * BrowserHistory* obj = new BrowserHistory(homepage);
+ * obj->visit(url);
+ * string param_2 = obj->back(steps);
+ * string param_3 = obj->forward(steps);
+ */