Answer 1

哈希表，时间复杂度O(n)

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

// const arr = [{id: 1}, {id:2, pId: 1}, {id: 3, pId: 2}, {id: 4}, {id:3, pId: 2}, {id: 5, pId: 4}]
// fn(arr) => [{id: 1, child: [{id: 2, pId: 1, child: [{ id: 3, pId: 2}]}]}, {id: 4, child: [{id: 5, pId: 4}]}]

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