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var reverseEvenLengthGroups = function (head) {
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  let groupSize = 2;
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  let start = head;
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  let prev = head;
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  let curr = head.next;
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  let count = 0;
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  while (curr != null) {
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    if (count === groupSize) {
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      if (groupSize % 2 === 0) {
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        // we only reverse when it is even
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        const end = curr;
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        const tail = start.next; // the starting node of the reverse linked list will be the tail after the reverse takes place
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        reverseList(start, end, count); // we need to reverse everything in the middle of start and end
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        start = tail; // we set the new start to the end of the reversed linked list
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      } else {
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        // when groupSize is even we don't need to reverse, but need to set the new start to the prev node
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        start = prev;
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      }
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      count = 0; // whenever we reached the group size we need to reset our count and up our groupSize
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      ++groupSize;
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    } else {
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      // just a normal traversal when we haven't hit our groupSize
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      prev = curr;
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      curr = curr.next;
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      ++count;
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    }
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  }
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  if (count % 2 === 0) {
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    // in the case where we ended early on even count
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    reverseList(start, null, count);
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  }
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  return head;
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  function reverseList(start, end, count) {
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    if (start.next == null) return start; // for case when we have a single node
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    let prev = start;
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    let curr = start.next;
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    let tail = start.next;
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    for (let i = 0; i < count; ++i) {
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      const next = curr.next;
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      curr.next = prev;
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      prev = curr;
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      curr = next;
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    }
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    start.next = prev;
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    tail.next = end;
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    return;
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  }
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};