SOURCE

function arrayToTreeIterative(items) {
const map = new Map();
const tree = []
//创建节点映射
items.forEach(item=>{
    map.set(item.id,{...item,children:[]})
})
//创建树结构
items.forEach(item=>{
    const node=map.get(item.id)
    if(item.parentId===null||item.parentId===undefined){
        tree.push(node)
    }else {
        const parent = map.get(item.parentId)
        if(parent){
            parent.children.push(node)
        }

    }
})
return tree
}

const flatArray = [
  { id: 1, name: "部门A", parentId: null },
  { id: 2, name: "部门B", parentId: 1 },
  { id: 3, name: "部门C", parentId: 1 },
  { id: 4, name: "部门D", parentId: 2 },
  { id: 5, name: "部门E", parentId: 2 },
  { id: 6, name: "部门F", parentId: 3 }
];
console.log(arrayToTreeIterative(flatArray))