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/**
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 * Definition for singly-linked list.
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 * function ListNode(val, next) {
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 *     this.val = (val===undefined ? 0 : val)
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 *     this.next = (next===undefined ? null : next)
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 * }
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 */
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/**
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 * @param {ListNode} head
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 * @param {number} left
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 * @param {number} right
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 * @return {ListNode}
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 */
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var reverseBetween = function (head, left, right) {
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  if (!head || left === right) {
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    return head;
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  }
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  const dummy = new ListNode(0, head);
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  let prev = dummy;
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  for (let i = 0; i < left - 1; i++) {
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    prev = prev.next;
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  }
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  let cur = prev.next;
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  for (let i = 0; i < right - left; i++) {
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    const temp = cur.next;
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    cur.next = temp.next;
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    temp.next = prev.next;
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    prev.next = temp;
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  }
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  return dummy.next;
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};