扁平数组转化为树状数组

const list= [
   {id: 1},
   {parentId: 1, id:2},
   {id: 3},
   {parentId: 3, id:4},
   {parentId:4, id:5}
]

const toTreeList = list => {
  // 1. 定义两个中间变量
  const treeList = [] 
  const map = {}      
  // 2. 添加一个空数组children
  list.forEach(item => {
    if (!item.children) {
      item.children = []
    }
    map[item.id] = item
  })
  // 3. 循环处理每个元素
  list.forEach(item => {
    const parent = map[item.parentId]
    if (parent) {
      parent.children.push(item)
    } else {
      treeList.push(item)
    }
  })
  console.log(treeList)
}
toTreeList(list)