From 7e9c6f13f596417262f2fb796c6ea8b815ef74c6 Mon Sep 17 00:00:00 2001
From: daretohack <cseanamika3@gmail.com>
Date: Sun, 30 May 2021 11:20:37 +0530
Subject: [PATCH] Priority Queue in Python

---
 Code/Python/priority_queue.py | 178 ++++++++++++++++++++++++++++++++++
 Queue/readme.md               |   1 +
 2 files changed, 179 insertions(+)
 create mode 100644 Code/Python/priority_queue.py

diff --git a/Code/Python/priority_queue.py b/Code/Python/priority_queue.py
new file mode 100644
index 000000000..428bc2794
--- /dev/null
+++ b/Code/Python/priority_queue.py
@@ -0,0 +1,178 @@
+""" Demonstartion of Priority Queue in Python.
+
+Priority Queue is an extension of queue with some additional properties-
+-> Every item has a priority associated with it.
+-> An element with high priority is dequeued before an element with low priority.
+-> If two elements have the same priority, they are served according to their order in the queue.
+
+A typical priority queue supports following operations:
+-> insert_into(item, priority): Inserts an item with given priority.
+-> get_min(): Returns the lowest priority item.
+-> remove_min(): Removes the lowest priority item.
+"""
+
+
+class PriorityQueueNode:
+    """
+    Node class for PriorityQueue.
+    """
+
+    def __init__(self, value, priority):
+        self.value = value
+        self.priority = priority
+
+
+class PriorityQueue:
+    """
+    PriorityQueue implementation using a list.
+    A priority is attached with every value of the queue.
+    """
+
+    def __init__(self):
+        self.pq = []
+
+    def get_size(self):
+        """
+        Return the size of the priority queue.
+        """
+        return len(self.pq)
+
+    def isEmpty(self):
+        """
+        Return boolean value.
+        `True` if the priority queue is empty,
+        esle, returns `False`.
+        """
+        return self.get_size() == 0
+
+    def get_min(self):
+        """
+        Return the least priority element in the priority queue.
+        """
+        if self.isEmpty():
+            return None
+        return self.pq[0].value
+
+    def insert_into_pq(self, value, priority):
+        """
+        Insert elements into the priority queue.
+        """
+        pq_node = PriorityQueueNode(value, priority)
+        self.pq.append(pq_node)
+
+        self.percolate_up()
+
+    def percolate_up(self):
+        """Find correct place for the element to be added
+        into the priority queue, following Heap Order Property.
+        """
+        child_index = self.get_size() - 1
+
+        while child_index > 0:
+            parent_index = (child_index - 1) // 2
+
+            if self.pq[child_index].priority < self.pq[parent_index].priority:
+                # swap
+                self.pq[child_index], self.pq[parent_index] = self.pq[parent_index], self.pq[child_index]
+                child_index = parent_index
+            else:
+                break
+
+    def remove_min(self):
+        """
+        Remove element with the least priority.
+        """
+        if self.isEmpty():
+            return None
+
+        ele = self.get_min()
+
+        self.pq[0] = self.pq[self.get_size() - 1]
+        self.pq.pop()
+
+        self.percolate_down()
+        return ele
+
+    def percolate_down(self):
+        """Find the position of the element to be removed
+        and ensures that Heap Order Property is maintained
+        after removal of the element.
+        """
+        parent_index = 0
+        left_child_index = (2 * parent_index) + 1
+        right_child_index = (2 * parent_index) + 2
+
+        while left_child_index < self.get_size():
+            min_index = parent_index
+
+            if self.pq[min_index].priority > self.pq[left_child_index].priority:
+                min_index = left_child_index
+
+            if right_child_index < self.get_size(
+            ) and self.pq[min_index].priority > self.pq[right_child_index].priority:
+                min_index = right_child_index
+
+            if min_index == parent_index:
+                break
+            self.pq[parent_index], self.pq[min_index] = self.pq[min_index], self.pq[parent_index]
+            parent_index = min_index
+            left_child_index = 2 * parent_index + 1
+            right_child_index = 2 * parent_index + 2
+
+
+if __name__ == "__main__":
+    pq1 = PriorityQueue()
+    size=int(input("Enter the number of values to initialise the priority queue with: "))
+    print("Enter values to push in the queue along with it's priority: ")
+    value=[]
+    for i in range(size):
+    	value.append([ele for ele in input().split()])
+    for i in range(len(value)):
+        pq1.insert_into_pq(value[i][0],value[i][1])
+    for i in range(pq1.get_size()):
+        print(pq1.remove_min(),end=" ")
+    print(pq1.isEmpty())
+
+"""Time Complexity Analysis:
+
+insert_into(): O(logN)
+
+get_min(): O(logN)
+
+remove_min(): O(logN)
+"""
+
+"""Test-Cases:
+
+>>>
+Input:
+Enter the number of values to initialise the priority queue with: 3
+Enter values to push in the queue along with it's priority:
+A 1
+B 2
+C 3
+
+Output:
+A
+B
+C
+True
+
+>>>
+Input:
+Enter the number of values to initialise the priority queue with: 5
+Enter values to push in the queue along with it's priority:
+Anamika a
+B.Tech b
+CSE c
+College c
+Python p
+
+Output:
+Anamika
+B.Tech
+College
+CSE
+Python
+True
+"""
diff --git a/Queue/readme.md b/Queue/readme.md
index cadf49b84..e5cf19678 100644
--- a/Queue/readme.md
+++ b/Queue/readme.md
@@ -44,3 +44,4 @@ called **overflow**. Generally, we treat them as exceptions. As an example, cons
 * Queue using Circular Linked List ----> [C++](/Code/C++/queue_using_circular_list.cpp)
 * Queue using Linkedlist ----> [C++](/Code/c++/Queue_all_operations_using_linkedlist.cpp)
 * Queue using Stacks ----> [C++](/Code/C++/queue_using_stacks.cpp)
+* Priority Queue ----> [Python](/Code/Python/priority_queue.py)
\ No newline at end of file