//dfs实现
var levelOrder = function (root) {
  let result = [];
  const dfs = (root, l) => {
    if (!root) return;
    if (result[l]) {
      result[l].push(root.val);
    } else {
      result[l] = [root.val];
    }
    dfs(root.left, l + 1);
    dfs(root.right, l + 1);
  };
  dfs(root, 0);
  return result;
};

//bfs 实现
var levelOrder = function (root) {
  if (!root) return [];
  const result = [];
  const q = [[root, 0]];
  while (q.length) {
    let [cur, l] = q.shift();
    if (!result[l]) {
      result[l] = [cur.val];
    } else {
      result[l].push(cur.val);
    }
    if (cur.left) q.push([cur.left, l + 1]);
    if (cur.right) q.push([cur.right, l + 1]);
  }
  return result;
};