Python Program to Solve Josephus Problem using Linked List

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This is a Python program to solve the Josephus problem using a linked list.

Problem Description

The program uses a circular single linked list to solve the Josephus problem.

Problem Solution

1. Create a class Node with instance variables data and next.
2. Create a class LinkedList with instance variable head.
3. The variable head points to the first element in the circular linked list.
4. Define methods get_node, get_prev_node, insert_after, insert_before, insert_at_end, remove and append.
5. The method get_node takes an index as argument and traverses the list from a specified node that many times to return the node at that index. The specified node is given by the second argument passed to it.
6. The method get_prev_node takes a reference node as argument and returns the previous node.
7. The methods insert_after and insert_before insert a node after or before some reference node in the list.
8. The methods insert_at_end inserts a node at the last position of the list.
9. The method remove takes a node as argument and removes it from the list.
10. The method appends a node with the data item passed to the end of the list.
11. Define function has_one_node which returns True only if the circular list passed to it has only one node.
12. Define function get_josephus_solution which takes a circular linked list and a number k as argument.
13. The function get_josephus_solution returns the solution to the Josephus problem where the people are represented by the nodes in the circular linked list and every kth person is executed.
14. Create an instance of CircularLinkedList and find the solution to the Josephus problem for a given k.

Program/Source Code

Here is the source code of a Python program to solve the Josephus problem using a linked list. The program output is shown below.

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None
 
 
class CircularLinkedList:
    def __init__(self):
        self.head = None
 
    def append(self, data):
        node = Node(data)
        self.insert_at_end(node)
 
    def get_node(self, index, start):
        if self.head is None:
            return None
        current = start
        for i in range(index):
            current = current.next
        return current
 
    def get_prev_node(self, ref_node):
        if self.head is None:
            return None
        current = self.head
        while current.next != ref_node:
            current = current.next
        return current
 
    def insert_after(self, ref_node, new_node):
        new_node.next = ref_node.next
        ref_node.next = new_node
 
    def insert_before(self, ref_node, new_node):
        prev_node = self.get_prev_node(ref_node)
        self.insert_after(prev_node, new_node)
 
    def insert_at_end(self, new_node):
        if self.head is None:
            self.head = new_node
            new_node.next = new_node
        else:
            self.insert_before(self.head, new_node)
 
    def remove(self, node):
        if self.head.next == self.head:
            self.head = None
        else:
            prev_node = self.get_prev_node(node)
            prev_node.next = node.next
            if self.head == node:
                self.head = node.next
 
 
def has_one_node(cllist):
    if cllist.head.next == cllist.head:
        return True
    else:
        return False
 
 
def get_josephus_solution(cllist, k):
    if cllist.head is None:
        return None
    start = cllist.head
    while not has_one_node(cllist):
        to_remove = cllist.get_node(k - 1, start)
        start = to_remove.next
        cllist.remove(to_remove)
    return cllist.head.data
 
 
a_cllist = CircularLinkedList()
n = int(input('Input number of people: '))
k = int(input('The kth person will be executed. Input k: '))
for i in range(1, n + 1):
    a_cllist.append(i)
 
ans = get_josephus_solution(a_cllist, k)
print('The person at position {} won\'t be killed.'.format(ans))
Program Explanation

1. The user is prompted to enter the values of n and k.
2. An instance of CircularLinkedList is created with n nodes where the nodes have values from 1 to n.
3. The function get_josephus_solution is called with the circular linked list and k as arguments.
4. The return value is the position of the person who won’t be executed in the Josephus problem.

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Runtime Test Cases
Case 1:
Input number of people: 5
The kth person will be executed. Input k: 3
The person at position 4 won't be killed.
 
Case 2:
Input number of people: 15
The kth person will be executed. Input k: 7
The person at position 5 won't be killed.
 
Case 3:
Input number of people: 8
The kth person will be executed. Input k: 2
The person at position 1 won't be killed.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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