将一个多维数组整合成树形结构，可以通过递归函数来实现

function buildTree(arr, parentId = null) {
  let tree = [];
  for (let i = 0; i < arr.length; i++) {
    let item = arr[i];
    if (item.parentId === parentId) {
      let children = buildTree(arr, item.id);
      if (children.length) {
        item.children = children;
      }
      tree.push(item);
    }
  }
  return tree;
}