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class Solution {
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  public ListNode swapNodes(ListNode head, int k) {
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    ListNode fast = head;
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    ListNode slow = head;
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    ListNode first = head, second = head;
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    // Put fast (k-1) nodes after slow
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    for (int i = 0; i < k - 1; ++i) fast = fast.next;
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    // Save the node for swapping
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    first = fast;
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    // Move until the end of the list
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    while (fast.next != null) {
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      slow = slow.next;
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      fast = fast.next;
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    }
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    // Save the second node for swapping
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    // Note that the pointer second isn't necessary: we could use slow for swapping as well
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    // However, having second improves readability
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    second = slow;
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    // Swap values
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    int temp = first.val;
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    first.val = second.val;
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    second.val = temp;
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    return head;
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  }
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}