SOURCE

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 },
];
function sort (arr) {
  var newArr = [];
  var conts = [];
  // 计数
  for (let i = 0; i < arr.length; i++) {
    var val = arr[i];
    if(!conts[val.pid]){
      conts[val.pid] = [];
    }
    conts[val.pid].push(val);
  }
//   console.log(conts);
  // 排序
  for (let j = 0; j < arr.length; j++) {
    var val = arr[j];
    if(conts[j]){
      val.children = conts[j];
    }
    if(val.pid === 0){
      newArr.push(val);
    }
  }
  return newArr;
}
var a = sort(arr);
console.log(a);