1669. 合并两个链表 #

var mergeInBetween = function(list1, a, b, list2) {
  let p1 = list1, p2 = list2, idx = 0
  while (idx < a - 1) {
    p1 = p1.next
    idx++
  }
  const prev = p1
  while (idx < b + 1) {
    p1 = p1.next
    idx++
  }

  while (p2.next) {
    p2 = p2.next
  }
  prev.next = list2
  p2.next = p1
  return list1
};