Is there something wrong with my reverse a linked list function ? It does not print the reversed lin-CodePudding

#include "bits/stdc  .h"
using namespace std;
struct ListNode
{
    int val ;
    ListNode * next;
    ListNode( int c )
    {
        val = c;
        next = NULL;
    }
};
void traversal ( ListNode * head )
{
    ListNode*temp = head ;
    while ( temp != NULL)
    {
        cout << temp->val << " ";
        temp=temp->next;
    }
    cout<<endl;
}
ListNode* reverseList(ListNode* head) 
{
    ListNode * temp=head;
    ListNode *p=head; 
    ListNode *x=NULL ;
    while( head->val  != x-> val )
    {
        while( temp->next != x)
            temp=temp->next;
        
        swap(head->val,temp->next->val);
        x=temp->next;
        head=head->next;
        temp= head ;

    }
    return p;
}
int main()
{
    ListNode * head = new ListNode ( 10 );
    head->next = new ListNode ( 20 );
    head-> next -> next =new ListNode ( 30 );
    head-> next -> next-> next =new ListNode ( 40 );
    head-> next -> next-> next-> next =new ListNode ( 50 );
    traversal ( head ) ;
    head = reverseList ( head ) ;
    traversal ( head ) ;

    return 0 ;
}

OUTPUT:
10 20 30 40 50 The program ends after printing this. I'm trying to reverse a singly linked list. I know this is not the right algorithm, I'm just creating my own algo to reverse it.I'm trying to maintain 2 pointer to swap the last and the first element till we reach the middle where the head -> data == x-> data . I'm using x pointer to store the address of the nth node ( in this case say address of 50 ) so that the next time my list iterates till over the second last element.The problem is that my list is not getting printed when i call the function.

CodePudding user response：

Basically this is what you need to do:

ListNode* reverseList(ListNode* head)
{
    ListNode *current = head;
    ListNode *previous = NULL;
    while (current != NULL) {
        ListNode *next = current->next;
        current->next = previous;
        previous = current;
        current = next;
    }
    head = previous;
    return head;
}

Your while was looping with the condition of ( head->val != x-> val ), and temp is initialized with head, so the two values will always match and as a result, the while condition will never be true.

CodePudding user response：

ListNode* reverseList(ListNode* head) 
{
    ListNode * temp=head, *p=head, *x=head;
    
    while ( x->next !=NULL)
        x=x->next;

    while( head->val  != x-> val )
    {
        while( temp->next != x)
        {
            temp=temp->next;
        }
        swap(head->val,temp->next->val);
        x=temp;
        head=head->next;
        temp= head ;
    }
    return p;
}

Fixed it :)