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var deleteDuplicates = function (head) {
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  const dummy = new ListNode();
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  dummy.next = head;
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  let node = dummy; // the last known distinct node
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  while (node.next) {
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    if (node.next.next && node.next.val === node.next.next.val) {
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      // if the next two nodes are equal...
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      let nonValNode = node.next.next.next;
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      while (nonValNode && nonValNode.val === node.next.val) {
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        // ...find the first one that isn't...
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        nonValNode = nonValNode.next;
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      }
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      node.next = nonValNode; // ...and glue it to the last known distinct node;...
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    } else {
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      node = node.next; // ...otherwise the next node is distinct
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    }
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  }
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  return dummy.next;
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};