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class Solution {
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  public ListNode swapPairs(ListNode head) {
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    ListNode dummy = new ListNode(0), prev = dummy, curr = head;
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    dummy.next = head;
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    while (curr != null && curr.next != null) {
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      prev.next = curr.next;
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      curr.next = curr.next.next;
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      prev.next.next = curr;
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      curr = curr.next;
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      prev = prev.next.next;
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    }
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    return dummy.next;
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  }
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}