1. 将[{id:1},{id:2, pId:1},...]转成[id1:child:[{id:2, pid:1}]]

const fn = arr => {
  const res = []
  const map = arr.reduces((res, item) => ((res[item.id] = item), res), {})
  for (const item of Object.values(map)) {
    it(!item.pId) {
      res.push(item)
    } else {
      const parent = map[item.pId]
      parent.child = patent.child || []
      parent.child.push(item)
    }
  }
  return res;
}