C++ reordering Singly Linked List

Question

Hi I'm trying to write a code for singly linked list that reorders the nodes so that:

L1->L2->L3->...->Ln to L1->Ln->L2->Ln-1->L3->Ln-2...

So I tried to do this by finding the node at the end of the list and setting that node as the next of current node and then finishing the loop by setting the current node as the next next of current node.

#include <cstdlib>
#include <iostream>

using namespace std;

struct ListNode {
        int val;
        ListNode *next;
        ListNode() : val(0), next(nullptr) {}
        ListNode(int x) : val(x), next(nullptr) {}
        ListNode(int x, ListNode *next) : val(x), next(next) {}
};

ListNode* newNode(int x){
    ListNode* temp = new ListNode;
    temp->val = x;
    temp->next = NULL;
    return temp;
}

void printlist(ListNode* head)
{
    while (head != NULL) {
        cout << head->val << " ";
        if (head->next)
            cout << "-> ";
        head = head->next;
    }
    cout << endl;
}

void reorderList(ListNode* head){
    ListNode *curr = head;
    ListNode *temp=head;
    ListNode *last=NULL;
    while(curr->next != NULL){
        while(temp->next != NULL){
            temp=temp->next;
            last=temp;
        }
        curr->next=last;
        last->next=curr->next->next;
        curr=curr->next->next;
        temp=curr->next;
    }
}

int main(int argc, char** argv) {
    ListNode* head = newNode(1);
    head->next = newNode(2);
    head->next->next = newNode(3);
    head->next->next->next = newNode(4);
    head->next->next->next->next = newNode(5);
 
    printlist(head); // Print original list
    reorderList(head); // Modify the list
    printlist(head); // Print modified list

    return 0;
}

So far, after displaying the original list, the program stops by saying that the run failed. I'm having some problem understanding the singly linked list and I don't know what I'm doing wrong.

Answer 1

I have modified your code to show a correct answer:

#include <iostream>

struct ListNode
{
  int val;
  ListNode* next;
  ListNode(): val( 0 ), next( nullptr ) {}
  ListNode( int x ): val( x ), next( nullptr ) {}
  ListNode( int x, ListNode* next ): val( x ), next( next ) {}
};

void printlist( ListNode* head )
{
  while( head != nullptr ) {
    std::cout << head->val << " ";
    if( head->next )
      std::cout << "-> ";
    head = head->next;
  }
  std::cout << std::endl;
}

void reorderList( ListNode* head ) {
  ListNode* pf = head;
  ListNode* pt = nullptr;
  ListNode* tmp = nullptr;
  while( true )
  {
    // find n-1 node
    tmp = pf;
    while( tmp && tmp->next && tmp->next->next )
      tmp = tmp->next;
    // check to see n-1 node is not equal to the first node
    if( tmp == pf )
      break;
    // reorder
    pt = tmp;
    tmp = pf->next;
    pf->next = pt->next;
    pt->next->next = tmp;
    pf = tmp;
    pt->next = nullptr;
  }
}

int main( int argc, char** argv ) {
  ListNode* head = new ListNode( 1 ); 
  head->next = new ListNode( 2 );
  head->next->next = new ListNode( 3 );
  head->next->next->next = new ListNode( 4 );
  head->next->next->next->next = new ListNode( 5 );
  head->next->next->next->next->next = new ListNode( 6 );
  printlist( head ); // Print original list
  reorderList( head ); // Modify the list
  printlist( head ); // Print modified list

  return 0;
}

and the result is like below:

1 -> 2 -> 3 -> 4 -> 5 -> 6                                                                                              
1 -> 6 -> 2 -> 5 -> 3 -> 4  

Answer 2

There could be many solutions to your problem. I just modified your logic and added comments which could help you to understand. Happy link list coding.

#include <cstdlib>
#include <iostream>
    
using namespace std;

struct ListNode {
        int val;
        ListNode *next;
        ListNode() : val(0), next(nullptr) {}
        ListNode(int x) : val(x), next(nullptr) {}
        ListNode(int x, ListNode *next) : val(x), next(next) {}
};

ListNode* newNode(int x){
    ListNode* temp = new ListNode;
    temp->val = x;
    temp->next = NULL;
    return temp;
}

void printlist(ListNode* head)
{
    while (head != NULL) {
        cout << head->val << " ";
        if (head->next)
            cout << "-> ";
        head = head->next;
    }
    cout << endl;
}

void reorderList(ListNode* head){
    ListNode *curr = head;
    ListNode *temp=head;
    ListNode *last=NULL,*prev;
    while(curr->next != NULL){
        
        while(temp->next != NULL){
            //Prev variable is being used for remove last node connection from it previous node
            // For example 1->2->3
            // We need to remove 2->3 connection before adding 1->3
            // Otherwise it will create cycle i.e 1->3->2->3->2....
            prev = temp;
            temp=temp->next;
            last=temp;
            
        }
        // If node number is even this condition will check adding mid+1 node twice
        if(last==NULL) {
            break;
        }
        temp = curr->next;
        curr->next=last;
        last->next=temp;
        curr=curr->next->next;
        temp=curr->next;
        // Removing last node connection from it previous node
        prev->next = NULL;
        last = NULL;
    }
}

int main(int argc, char** argv) {
    ListNode* head = newNode(1);
    head->next = newNode(2);
    head->next->next = newNode(3);
    head->next->next->next = newNode(4);
    head->next->next->next->next = newNode(5);
   //head->next->next->next->next->next = newNode(8);
 
    printlist(head); // Print original list
    reorderList(head); // Modify the list
    printlist(head); // Print modified list

    return 0;
}