Reverse a linked list

Problem Statement :

Given the pointer to the head node of a linked list, change the next pointers of the nodes so that their order is reversed. The head pointer given may be null meaning that the initial list is empty.

 head references the list 1->2->3->Null.

Manipulate the next pointers of each node in place and return head, now referencing the head of the list 3->2->1->Null.

Function Description:

Complete the reverse function in the editor below.

reverse has the following parameter:
           SinglyLinkedListNode pointer head: a reference to the head of a list
           SinglyLinkedListNode pointer: a reference to the head of the reversed list

Input Format:

The first line contains an integer t, the number of test cases.
Each test case has the following format:
       1. The first line contains an integer n, the number of elements in the linked list.
       2. Each of the next n lines contains an integer, the data values of the elements in the linked list.

       1.   1<=t<=10
       2.   1<=n<=1000
       3.   1<=list[i]<=1000

Solution :


                            Solution in C :

In C:

//the following function is all that is needed to complete the
//challenge in hackerrank platform.

SinglyLinkedListNode* reverse(SinglyLinkedListNode* head) {
    struct SinglyLinkedListNode* new_head = NULL;
    struct SinglyLinkedListNode* temp;
    while(head != NULL){
        temp = head->next;
        head->next = new_head;
        new_head = head;
        head = temp;
    return new_head;


In C++:

//the following function is all that is needed to complete the
//challenge in hackerrank platform.

Node* Reverse(Node *head)
  // Complete this method
    Node* prev   = NULL;
    Node* current = head;
    Node* next;
    while (current != NULL)
        next  = current->next;  
        current->next = prev;   
        prev = current;
        current = next;
    return prev;

In Java:

//the following method is all that is needed to complete the
//challenge in hackerrank platform.

  static SinglyLinkedListNode reverse(SinglyLinkedListNode head) {
        SinglyLinkedListNode next = null;
        SinglyLinkedListNode current = head;
        SinglyLinkedListNode previous = null;
            next =;
   = previous;
            previous = current;
            current = next;
        SinglyLinkedListNode singlyLinkedListNode = previous;
        return singlyLinkedListNode;

In Python 3:

//the following method is all that is needed to complete the
//challenge in hackerrank platform.

def Reverse(head):
    current = head
    prev = None
    next = None
    while current is not None:
        next = = prev;  
        prev = current;
        current = next;
    return prev

View More Similar Problems

Self-Driving Bus

Treeland is a country with n cities and n - 1 roads. There is exactly one path between any two cities. The ruler of Treeland wants to implement a self-driving bus system and asks tree-loving Alex to plan the bus routes. Alex decides that each route must contain a subset of connected cities; a subset of cities is connected if the following two conditions are true: There is a path between ever

View Solution →

Unique Colors

You are given an unrooted tree of n nodes numbered from 1 to n . Each node i has a color, ci. Let d( i , j ) be the number of different colors in the path between node i and node j. For each node i, calculate the value of sum, defined as follows: Your task is to print the value of sumi for each node 1 <= i <= n. Input Format The first line contains a single integer, n, denoti

View Solution →

Fibonacci Numbers Tree

Shashank loves trees and math. He has a rooted tree, T , consisting of N nodes uniquely labeled with integers in the inclusive range [1 , N ]. The node labeled as 1 is the root node of tree , and each node in is associated with some positive integer value (all values are initially ). Let's define Fk as the Kth Fibonacci number. Shashank wants to perform 22 types of operations over his tree, T

View Solution →

Pair Sums

Given an array, we define its value to be the value obtained by following these instructions: Write down all pairs of numbers from this array. Compute the product of each pair. Find the sum of all the products. For example, for a given array, for a given array [7,2 ,-1 ,2 ] Note that ( 7 , 2 ) is listed twice, one for each occurrence of 2. Given an array of integers, find the largest v

View Solution →

Lazy White Falcon

White Falcon just solved the data structure problem below using heavy-light decomposition. Can you help her find a new solution that doesn't require implementing any fancy techniques? There are 2 types of query operations that can be performed on a tree: 1 u x: Assign x as the value of node u. 2 u v: Print the sum of the node values in the unique path from node u to node v. Given a tree wi

View Solution →

Ticket to Ride

Simon received the board game Ticket to Ride as a birthday present. After playing it with his friends, he decides to come up with a strategy for the game. There are n cities on the map and n - 1 road plans. Each road plan consists of the following: Two cities which can be directly connected by a road. The length of the proposed road. The entire road plan is designed in such a way that if o

View Solution →