1
class Solution {
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  public ListNode reverseKGroup(ListNode head, int k) {
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    int numOfNodes = count(head);
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    ListNode ptr = null;
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    List<ListNode> start = new ArrayList<>(), end = new ArrayList<>();
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    ListNode f = null;
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    while (head != null) {
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      if (numOfNodes >= k) {
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        start.add(head);
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        int count = 0;
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        while (count < k) {
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          ListNode temp = head.next;
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          head.next = ptr;
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          ptr = head;
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          head = temp;
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          count++;
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        }
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        end.add(ptr);
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        ptr = null;
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        numOfNodes -= count;
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      } else {
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        f = head;
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        break;
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      }
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    }
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    int n = start.size();
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    for (int i = 0; i < n - 1; i++) start.get(i).next = end.get(i + 1);
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    start.get(n - 1).next = f;
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    return end.get(0);
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  }
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  public int count(ListNode head) {
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    ListNode temp = head;
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    int count = 0;
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    while (temp != null) {
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      count++;
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      temp = temp.next;
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    }
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    return count;
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  }
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}