2.27 数组转换为树形结构

1.for循环

      function toTree(arr, parentId) {
        function loop(parentId) {
          let res = []
          for (let i = 0; i < arr.length; i++) {
            let item = arr[i]
            if (item.pid !== parentId) {
              continue
            }
            item.children = loop(item.id)
            res.push(item)
          }
          return res
        }
        return loop(parId)
      }