关于javascript:题目扁平数据结构转Tree

面试了十几个高级前端，居然连（扁平数据结构转 Tree）都写不进去

具体题目详情点击下面查看，树转扁平做得多，扁平转树还真没写过。有意思。上午看到做个笔迹记录下。
扁平数据如下：

let arr = [{id: 1, name: '部门 1', pid: 0},
    {id: 2, name: '部门 2', pid: 1},
    {id: 3, name: '部门 3', pid: 1},
    {id: 4, name: '部门 4', pid: 3},
    {id: 5, name: '部门 5', pid: 4},
]

输入后果如下，有限层级的树：

[
    {
        "id": 1,
        "name": "部门 1",
        "pid": 0,
        "children": [
            {
                "id": 2,
                "name": "部门 2",
                "pid": 1,
                "children": []},
            {
                "id": 3,
                "name": "部门 3",
                "pid": 1,
                "children": [// 后果 ,,,]
            }
        ]
    }
]

本人写的解答办法

const flap2Tree = (flapArr)=>{
    // 递归依据 pid 找父
    const findByPid = (pid, data)=> {for(let i=0; i<data.length; i++){if( data[i].id === pid ) {return data[i];
           }else if(data[i].children && data[i].children.length){return pidFindParent(pid, data[i].children);
           }
        }
    }

    let resTree = [];
    flapArr.forEach( t => {let myParent = findByPid(t.pid, afterArr); 
        if(myParent){myParent.children || (myParent.children = []); // 初始化
            myParent.children.push(t);
        }else{resTree.push(t);
        }
    })
    return resTree;
}
flap2Tree(arr);

学习博主最优性能办法

function arrayToTree(items) {const result = [];   // 寄存后果集
  const itemMap = {};  // 
  for (const item of items) {
    const id = item.id;
    const pid = item.pid;

    if (!itemMap[id]) {itemMap[id] = {children: [],
      }
    }

    itemMap[id] = {
      ...item,
      children: itemMap[id]['children']
    }

    const treeItem =  itemMap[id];

    if (pid === 0) {result.push(treeItem);
    } else {if (!itemMap[pid]) {itemMap[pid] = {children: [],
        }
      }
      itemMap[pid].children.push(treeItem)
    }

  }
  return result;
}