劍指offer：從上到下打印二叉樹

題目描述
從上往下打印出二叉樹的每個節點，同層節點從左至右打印。

class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution:
    """
    由于需要逐層打印，那么我們在遍歷整棵樹的時候就需要維護一個隊列。
    隊列中存儲的是下一層從左到右的節點。
    具體來說在打印第k層的節點的時候，將該節點的左右子節點按順序入隊即可。遞歸出口就是隊列為空
    """
    def PrintFromTopToBottom(self, root):
        def helper(root_queue, ans):
            # 遞歸出口即隊列為空
            if not root_queue:
                return
            length = len(root_queue)
            for i in range(length):
                # 對于某個節點，在打印完它的值之后，將其左右子節點先后入隊
                ans.append(root_queue[i].val)
                if root_queue[i].left:
                    root_queue.append(root_queue[i].left)
                if root_queue[i].right:
                    root_queue.append(root_queue[i].right)

            helper(root_queue[length:], ans)

        res = []
        if not root:
            return res
        helper([root], res)
        return res

def main():
    root = TreeNode(8)
    root.left = TreeNode(6)
    root.right = TreeNode(10)
    root.left.left = TreeNode(5)
    root.left.right = TreeNode(7)
    root.right.left = TreeNode(9)
    root.right.right = TreeNode(11)

    solution = Solution()
    print(solution.PrintFromTopToBottom(root))

if __name__ == '__main__':
    main()