024.Swap Nodes In Pairs

Test cases

[]
[1]
[1,2]
[1,2,3]
[1,2,3,4]
[1,2,3,4,5]

Solution1: accepted

Primitive brute solution. 1ms but is below 99%.

public class Solution {
    public ListNode swapPairs(ListNode head) {
        if (head == null || head.next == null)
            return head;
            
        ListNode result = new ListNode(0);
        ListNode operation = result;
        ListNode even = head.next;
        ListNode odd = head;
        boolean terminate = false;
        while (even != null || odd != null) {
            
            operation.next = new ListNode(even.val);
            operation = operation.next;
            
            if (even.next != null && even.next.next != null) 
                even = even.next.next;
            else 
                terminate = true;
                
            operation.next = new ListNode(odd.val);
            operation = operation.next;
            
            if (odd.next != null && odd.next.next != null) 
                odd = odd.next.next;
            else 
                terminate = true;
            
            if (terminate)
                break;
            // System.out.println(head);
        }
        if (odd != null && odd.next == null)
            operation.next = new ListNode(odd.val);
            
        return result.next;
        
    }
}

Solution 2: accepted

Bring forward the second number and remove it from the first listnode

public class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode operator = dummy;
        
        while(operator.next != null && operator.next.next != null) {
            ListNode odd = operator.next;
            ListNode even = operator.next.next;
            operator.next = even;
            odd.next = even.next; // Remove the used additional even
            operator.next.next = odd;
            operator = operator.next.next;
        }
        return dummy.next;
    }
}

Solution 3: accepted

public class Solution {
    public ListNode swapPairs(ListNode head) {
       if(head == null || head.next == null) return head;
        ListNode nextNode = head.next;
        head.next = swapPairs(nextNode.next);
        nextNode.next = head;
        return nextNode;
    }
}