CareerCup

swap first two nodes and then call recursively for the rest of the linked list

void swapaltrec(list *head)
{
    list p=*head,q;
    if(!p || !(p->next))
    return;
    q=p->next;
    p->next=q->next;
    q->next=p;
    *head=q;
    swapaltrec(&(p->next));
}

void swapNode(list *head)
{
	list *temp, *temp1;
	temp = head->next;
	while (head && temp)
	{
		temp1 = temp->next;
		head->next = temp1;
		temp->next = head;

		head = temp1->next;
		temp = head->next;
	}
}

procedure(Node start){
	
	if(start == null) return;

        Node x = start;
        Node y = x.next;

	while(x!= null and y != null)
	{

		x.next = y.next;
		y.next = x;
		
		x = x.next;
		if(x == null) return;
		y = x.next;
	}
}

This code still has issues since we are not modifying the start of the linked list which will get modified too in case of swapping 1st and 2nd node of the linked list

procedure(Node start){
	
	if(start == null) return;

	boolean startNodeModifed = false;
        Node x = start;
        Node y = x.next;

	while(x!= null and y != null)
	{
		
		
		x.next = y.next;
		y.next = x;
		if(!startNodeModifed) 		
			start = y;
			
		x = x.next;
		if(x == null) return;
		y = x.next;
	}
}

void swp(llist *head)
{
    llist *a,*b;
    if(head->next==NULL || head->next->next==NULL) return;
    a=head;
    b=head->next;

    int tmp=a->x;
    a->x=b->x;
    b->x=tmp;

    if(b->next!=NULL)
        swp(b->next);
    return;
}

Code:

void SwapNodes(Node** start)
{
	if (start == NULL || *start == NULL)
		return;

	Node* temp = *start;
	if (temp ->next != null)
	{
		*start = temp->next;
		while (temp && temp->next)
		{
			Node* next = temp->next->next;
			Node* temp2 = temp->next;
			temp2->next = temp;
			temp->next = next;
			temp = next;
		}
	}
}

typedef struct ListNode 
{ 
	int val;
	ListNode *next;
};


ListNode* swap(ListNode *head)
{
	if (!head) 
		return NULL;

	ListNode *node = head;
	ListNode *next = node->next;
	if (!next) 
		return head;
	
	head = next;
	ListNode *prev = NULL;
	do
	{
		node->next = next->next;
		next->next = node;
		if (prev)
			prev->next = next;
		prev = node;
  
	} while ((node=node->next) && (next=node->next));

	return head;
} 
void main(void)
{

	ListNode *h = new ListNode;
	h->val = 1; 
	h->next = NULL;

	ListNode *t = h;
	for (int i=2; i<=13; ++i)
	{
		t->next = new ListNode;
		t->val = i; 
		t = t->next;
	}
	t->next = NULL;

	ListNode *n = NULL; 
	for (n=h; n; n=n->next)
		cout << n->val << " " ;
	cout << endl;


	h = swap(h);
	for (n=h; n; n=n->next)
		cout << n->val << " " ;
	cout << endl;

	system("pause");
}