Select a Random Node from a Singly Linked List

Given a singly linked list, select a random node from linked list (the probability of picking a node should be 1/N if there are N nodes in list). You are given a random number generator.

Below is a Simple Solution
1) Count number of nodes by traversing the list.
2) Traverse the list again and select every node with probability 1/N. The selection can be done by generating a random number from 0 to N-i for i’th node, and selecting the i’th node node only if generated number is equal to 0 (or any other fixed number from 0 to N-i).

We get uniform probabilities with above schemes.

i = 1, probability of selecting first node = 1/N
i = 2, probability of selecting second node =
                   [probability that first node is not selected] * 
                   [probability that second node is selected]
                  = ((N-1)/N)* 1/(N-1)
                  = 1/N  

Similarly, probabilities of other selecting other nodes is 1/N

The above solution requires two traversals of linked list.

How to select a random node with only one traversal allowed?
The idea is to use Reservoir Sampling. Following are the steps. This is a simpler version of Reservoir Sampling as we need to select only one key instead of k keys.

(1) Initialize result as first node
   result = head->key 
(2) Initialize n = 2
(3) Now one by one consider all nodes from 2nd node onward.
    (3.a) Generate a random number from 0 to n-1. 
         Let the generated random number is j.
    (3.b) If j is equal to 0 (we could choose other fixed number 
          between 0 to n-1), then replace result with current node.
    (3.c) n = n+1
    (3.d) current = current->next

Below is the implementation of above algorithm.

C

/* C program to randomly select a node from a singly
   linked list */
#include<stdio.h>
#include<stdlib.h>
#include <time.h>

/* Link list node */
struct Node
{
    int key;
    struct Node* next;
};

// A reservoir sampling based function to print a
// random node from a linked list
void printRandom(struct Node *head)
{
    // IF list is empty
    if (head == NULL)
       return;

    // Use a different seed value so that we don't get
    // same result each time we run this program
    srand(time(NULL));

    // Initialize result as first node
    int result = head->key;

    // Iterate from the (k+1)th element to nth element
    struct Node *current = head;
    int n;
    for (n=2; current!=NULL; n++)
    {
        // change result with probability 1/n
        if (rand() % n == 0)
           result = current->key;

        // Move to next node
        current = current->next;
    }

    printf("Randomly selected key is %d\n", result);
}

/* BELOW FUNCTIONS ARE JUST UTILITY TO TEST  */

/* A utility function to create a new node */
struct Node *newNode(int new_key)
{
    /* allocate node */
    struct Node* new_node =
        (struct Node*) malloc(sizeof(struct Node));

    /* put in the key  */
    new_node->key  = new_key;
    new_node->next =  NULL;

    return new_node;
}


/* A utility function to insert a node at the beginning
  of linked list */
void push(struct Node** head_ref, int new_key)
{
    /* allocate node */
    struct Node* new_node = new Node;

    /* put in the key  */
    new_node->key  = new_key;

    /* link the old list off the new node */
    new_node->next = (*head_ref);

    /* move the head to point to the new node */
    (*head_ref)    = new_node;
}


// Driver program to test above functions
int main()
{
    struct Node *head = NULL;
    push(&head, 5);
    push(&head, 20);
    push(&head, 4);
    push(&head, 3);
    push(&head, 30);

    printRandom(head);

    return 0;
}

Java

// Java program to select a random node from singly linked list

import java.util.*;

// Linked List Class
class LinkedList {

    static Node head;  // head of list

    /* Node Class */
    static class Node {

        int data;
        Node next;

        // Constructor to create a new node
        Node(int d) {
            data = d;
            next = null;
        }
    }

    // A reservoir sampling based function to print a
    // random node from a linked list
    void printrandom(Node node) {

        // If list is empty
        if (node == null) {
            return;
        }

        // Use a different seed value so that we don't get
        // same result each time we run this program
        Math.abs(UUID.randomUUID().getMostSignificantBits());

        // Initialize result as first node
        int result = node.data;

        // Iterate from the (k+1)th element to nth element
        Node current = node;
        int n;
        for (n = 2; current != null; n++) {

            // change result with probability 1/n
            if (Math.random() % n == 0) {
                result = current.data;
            }

            // Move to next node
            current = current.next;
        }

        System.out.println("Randomly selected key is " + result);
    }

    // Driver program to test above functions
    public static void main(String[] args) {

        LinkedList list = new LinkedList();
        list.head = new Node(5);
        list.head.next = new Node(20);
        list.head.next.next = new Node(4);
        list.head.next.next.next = new Node(3);
        list.head.next.next.next.next = new Node(30);

        list.printrandom(head);

    }
}

// This code has been contributed by Mayank Jaiswal

Python


# Python program to randomly select a node from singly
# linked list 

import random

# Node class 
class Node:

    # Constructor to initialize the node object
    def __init__(self, data):
        self.data= data
        self.next = None

class LinkedList:

    # Function to initialize head
    def __init__(self):
        self.head = None

    # A reservoir sampling based function to print a
    # random node from a linkd list
    def printRandom(self):

        # If list is empty 
        if self.head is None:
            return 

        # Use a different seed value so that we don't get 
        # same result each time we run this program
        random.seed()

        # Initialize result as first node
        result = self.head.data

        # Iterate from the (k+1)th element nth element
        current = self.head 
        n = 2 
        while(current is not None):
            
            # change result with probability 1/n
            if (random.randrange(n) == 0 ):
                result = current.data 

            # Move to next node
            current = current.next
            n += 1

        print "Randomly selected key is %d" %(result)
        
    # Function to insert a new node at the beginning
    def push(self, new_data):
        new_node = Node(new_data)
        new_node.next = self.head
        self.head = new_node

    # Utility function to print the linked LinkedList
    def printList(self):
        temp = self.head
        while(temp):
            print temp.data,
            temp = temp.next


# Driver program to test above function
llist = LinkedList()
llist.push(5)
llist.push(20)
llist.push(4)
llist.push(3)
llist.push(30)
llist.printRandom()

# This code is contributed by Nikhil Kumar Singh(nickzuck_007)

Note that the above program is based on outcome of a random function and may produce different output.

How does this work?
Let there be total N nodes in list. It is easier to understand from last node.

The probability that last node is result simply 1/N [For last or N’th node, we generate a random number between 0 to N-1 and make last node as result if the generated number is 0 (or any other fixed number]

The probability that second last node is result should also be 1/N.

The probability that the second last node is result 
          = [Probability that the second last node replaces result] X 
            [Probability that the last node doesn't replace the result] 
          = [1 / (N-1)] * [(N-1)/N]
          = 1/N

Similarly we can show probability for 3rd last node and other nodes.

This article is contributed by Rajeev. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

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