顺序遍历：求解《1669. 合并两个链表》

var mergeInBetween = function(list1, a, b, list2) {
  let r = new ListNode(null, list1), i = 0
  while (++i < a) list1 = list1.next
  let next1 = list1.next
  list1.next = list2
  while (i++ <= b) next1 = next1.next
  while (list2.next) list2 = list2.next
  list2.next = next1
  return r.next
};

func mergeInBetween(list1 *ListNode, a int, b int, list2 *ListNode) *ListNode {
  r, i := &ListNode{Val: 0, Next: list1}, 0
  for i < a - 1 {
    list1 = list1.Next
    i++
  }
  next1 := list1.Next
  list1.Next = list2
  for i < b {
    next1 = next1.Next
    i++
  }
  for list2.Next != nil {
    list2 = list2.Next
  }
  list2.Next = next1
  return r.Next
}

class Solution {
  function mergeInBetween($list1, $a, $b, $list2) {
    $r = new ListNode(null, $list1);
    $i = 0;
    while (++$i < $a) $list1 = $list1->next;
    $next1 = $list1->next;
    $list1->next = $list2;
    while ($i++ <= $b) $next1 = $next1->next;
    while ($list2->next) $list2 = $list2->next;
    $list2->next = $next1;
    return $r->next;
  }
}