剑指 Offer 32 - I. 从上到下打印二叉树

BFS模板的应用。

总结的DFS、BFS模板参考：https://www.cnblogs.com/panweiwei/p/13065661.html

class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if not root:
            return []
        res = []
        stack = [root]
        while stack:
            sizestack = len(stack)
            for i in range(sizestack):
                node = stack.pop(0)
                res.append(node.val)
                if node.left:
                    stack.append(node.left)
                if node.right:
                    stack.append(node.right)
        return res