打卡群2刷题总结1010——二叉树的层序遍历

示例：
二叉树：[3,9,20,null,null,15,7],
	3
   / \
  9  20
    /  \
   15   7
返回其层次遍历结果：

[
  [3],
  [9,20],
  [15,7]
]

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        queue = [root]
        res = []
        # 只要不为空，就得继续遍历
        while len(queue) > 0:
            tmp = []
            res_tmp = []
            # 遍历所有元素
            while len(queue) > 0:
                node = queue.pop(0)
                if not node:
                    continue
                res_tmp.append(node.val)
                tmp.append(node.left)
                tmp.append(node.right)

            if len(res_tmp) > 0:
                queue = tmp
                res.append(res_tmp)
        return res