将一个扁平数组转换成树状结构

function toChildrenStruct(pidArray, expandLevel) {
    // 将pid结构的树型数据转换成children结构
    const childrenArray = []
    const expandedItems = []
    const itemMap = {}
    if (!pidArray) {
      return childrenArray
    } else {
      for (const item of pidArray) {
        item.key = item.id
        item.value = item.id
        item.label = item.name
        itemMap[item.id] = item
      }
      for (const item of pidArray) {
        const parent = itemMap[item.parentId]
        if (typeof parent === 'undefined') {
          // pid不存在，是顶级元素
          childrenArray.push(item)
        } else {
          if (typeof parent.children === 'undefined') {
            parent.children = []
            if (expandLevel === -1) expandedItems.push(parent.id)
          }
          parent.children.push(item)
        }
      }
    }
    return childrenArray
  }