24. Swap Nodes in Pairs

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Recursion

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

        // nodeA -> nodeB -> ...
        ListNode nodeA = head, nodeB = head.next;
        ListNode nextHead = nodeB.next;

        nodeB.next = nodeA;
        nodeA.next = swapPairs(nextHead);
        return nodeB;
    }
}

Iterate

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class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode dummyHead = new ListNode(-1, head);
        ListNode pre = dummyHead;

        ListNode curr = head;
        while (curr != null && curr.next != null) {
            ListNode nodeA = curr, nodeB = curr.next;
            ListNode nextCurr = nodeB.next;

            nodeB.next = nodeA;
            nodeA.next = nextCurr;

            pre.next = nodeB;

            curr = nextCurr;
            pre = nodeA;
        }
        
        return dummyHead.next;
    }
}

References

24. Swap Nodes in Pairs