added Priority Queue

code/data_structures/src/queue/priority_queue/README.md

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+# Priority Queue
+
+## Description
+
+A priority queue is a collection in which items can be added at any time, but the only item that can be removed is the one with the highest priority.
+
+## Functions
+- add(x,p)
+-- add item ```x``` with priority ```p```.
+- remove
+-- remove the highest priority item.
+- peek
+-- return the highest priority item (without removing it).
+- size
+-- return the number of items in the priority queue.
+- isEmpty
+-- return whether the priority queue has no items.
+
+## Time Complexity
+
+- Addition : O(n)
+- Remove   : O(1)
+- Peek     : O(1)

code/data_structures/src/queue/priority_queue/priority_queue.cpp

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+// C++ implementation of Priority Queue
+
+#include <iostream>
+#include <cstdio>
+#include <cstring>
+#include <cstdlib>
+
+// Node Declaration
+struct node {
+	int priority;
+	int info;
+	node *link;
+};
+
+class PriorityQueue {
+    private:
+        node *front;
+    public:
+        PriorityQueue() {
+            front = nullptr;
+        }
+        // Insert into Priority Queue
+        void add(int item, int priority) {
+            node *tmp, *q;
+            tmp = new node;
+            tmp->info = item;
+            tmp->priority = priority;
+            if (front == nullptr || priority < front->priority){
+                tmp -> link = front;
+                front = tmp;
+            }
+            else {
+                q = front;
+                while (q->link != nullptr && q->link -> priority <= priority)
+                    q = q->link;
+
+                tmp->link = q->link;
+                q->link = tmp;
+            }
+        }
+
+        // Delete from Priority Queue
+        void remove() {
+            node *tmp;
+            if(front == nullptr)
+                std::cout << "Queue Underflow\n";
+            else {
+                tmp = front;
+                std::cout << "Removed item is: " << tmp->info << "\n";
+                front = front->link;
+                free(tmp);
+            }
+        }
+
+        // Print Priority Queue
+        void display() {
+            node *ptr;
+            ptr = front;
+            if (front == nullptr)
+                std::cout << "Queue is empty\n";
+            else{	
+                std::cout << "Queue is :\n";
+                std::cout << "Priority Item\n";
+                while(ptr != nullptr){
+                    std::cout << ptr->priority << "\t\t" << ptr->info << "\n";
+                    ptr = ptr->link;
+                }
+            }
+        }
+};
+
+int main() {
+
+    int choice, item, priority;
+    PriorityQueue pq; 
+
+    do{
+        std::cout << "1.Insert\n";
+        std::cout << "2.Delete\n";
+        std::cout << "3.Display\n";
+        std::cout << "4.Quit\n";
+        std::cout << "Enter your choice : "; 
+        std::cin >> choice;
+        switch(choice){
+        case 1:
+            std::cout << "Input the item value : ";
+            std::cin >> item;
+            std::cout << "Enter its priority : ";
+            std::cin >> priority;
+            pq.add(item, priority);
+            break;
+        case 2:
+            pq.remove();
+            break;
+        case 3:
+            pq.display();
+            break;
+        case 4:
+            break;
+        default :
+            std::cout << "Wrong Choice !!!\n";
+        }
+    }
+    while(choice != 4);
+
+    return 0;
+}