SOURCE

const arr = [
    { id: 2, pid: 1,name:'bb' },
    { id: 3, pid: 1,name:'cc' },
    { id: 4, pid: 2,name:'dd' },
    { id: 1, pid: 0,name:'aa' },
    { id: 5, pid: 2,name:'ee' },
    { id: 6, pid: 3,name:'ff' },
];

function arr2tree (arr) {
  // parentHash用于确定rootId,childHash用于构建树形结构
  //步骤一：转化数组数据称为hash结构数据
  const parentHash = {}, childHash = {};
  for (let item of arr) {
    const { id, pid,name } = item;
    parentHash[id] = {pid:pid,name:name};

    if (!childHash[pid]) {
      childHash[pid] = [];
    }
    childHash[pid].push({id:id,name:name});
  }
  console.log('p=',parentHash);
  console.log('c=',childHash)

  //步骤二：确定树形结构的根节点
  let rootId = arr[0].id;
  while (parentHash[rootId] !== undefined) {
    rootId = parentHash[rootId];
  }
  console.log('root=',rootId)

  //步骤三：将hash结构转化为树形结构
  return hash2Tree(childHash, rootId);

}

//函数作用：将hash结构转化为树形结构
function hash2Tree (childHash, rootId) {
  let res = { id: rootId };
  if (childHash[rootId] && childHash[rootId].length > 0) {
    res.children = childHash[rootId].map((cid) => hash2Tree(childHash, cid))
  }
  return res;
}
let a = arr2tree(arr);
console.log(a)