function convert(data) {
  const m = new Map();
  data.forEach(node => {
    m.set(node.id, node);
    node.children = [];
  });

  data.forEach(node => {
    const parent = m.get(node.parentId);
    if (parent) {
      parent.children.push(node);
    }
  });
}

用map保存节点和id，O(n)搞定