old_contents: from TreeNode import TreeNode
  class Solution(object):
      def levelOrder(self, root):
          res = []
          if root :
              q = [root]
              i = 0
              while i < len(q) :
                  level = []
                  for j in range(len(q) - i) :
                      node = q[i]
                      level.append(node.val)
                      if node.left:
                          q.append(node.left)
                      if node.right:
                          q.append(node.right)
                      i += 1
                  res.append(level)
          return res
new_contents: # https://leetcode.com/problems/binary-tree-level-order-traversal/
  from TreeNode import TreeNode
  class Solution(object):
      def levelOrder(self, root):
          res = []
          if root :
              q = [root]
              i = 0
              while i < len(q) :
                  level = []
                  for j in range(len(q) - i) :
                      node = q[i]
                      level.append(node.val)
                      if node.left:
                          q.append(node.left)
                      if node.right:
                          q.append(node.right)
                      i += 1
                  res.append(level)
          return res
subject: add describe