1
class Solution {
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  public ListNode reverseBetween(ListNode head, int left, int right) {
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    if (left == right) return head;
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    ListNode last = null;
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    ListNode present = head;
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    for (int i = 0; present != null && i < left - 1; i++) {
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      last = present;
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      present = present.next;
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    }
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    ListNode finalEnd = present;
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    ListNode prev = last;
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    ListNode next = present.next;
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    for (int i = 0; present != null && i < right - left + 1; i++) {
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      present.next = prev;
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      prev = present;
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      present = next;
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      if (next != null) {
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        next = next.next;
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      }
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    }
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    if (last != null) {
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      last.next = prev;
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    } else {
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      head = prev;
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    }
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    finalEnd.next = present;
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    return head;
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  }
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}