二叉树的层序遍历 #7

var levelOrder = function(root) {
  if(!root) {
      return []
  }
  let q = [[root, 0]]
  let arr = []
  while(q.length) {
      let [n, level] = q.shift()
      if(!arr[level]) {
        arr[level] = [n.val] 
      } else {
          arr[level].push(n.val)
      }
      if(n.left) q.push([n.left, level + 1])
      if(n.right) q.push([n.right, level + 1])
  }
  return arr
};