leetcode——102. 二叉树的层次遍历

class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        levels=[]
        if not root:
            return levels
        def helper(node,level):
            if len(levels)==level:
                levels.append([])
            levels[level].append(node.val)
            if node.left:
                helper(node.left,level+1)
            if node.right:
                helper(node.right,level+1)

        helper(root,0)
        return levels