與上一篇部落格層次遍歷二叉樹不同，這次是需要將每一層列印在同一行，這就需要判斷每一層的元素：
注意到，將每一層列印完了之後，下面的一層也全部進入到了佇列，因此採用一個計數器：nextLevel來計算下一層的個數，使用toBeList計算這層還麼有列印完的個數
初始nextLevel = 0， toBeList = 1，之後每次放入一個佇列，nextLevel+1，每次出一個佇列，toBeList-1，知道toBeList為0之後，就說明這層的列印完了，同時下一層已經計數了

            if toBeList == 0:
                results.append(
 
result)
                result = []
                toBeList = nextLevel
                nextLevel = 0

程式碼：

class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None
class Solution:
    # 返回從上到下每個節點值列表，例：[1,2,3]
    def PrintFromTopToBottom
 
(self, root):
        # write code here
        list = []  #佇列
        result = []  #每一層的結果
        results = []  #儲存result的list
        nextLevel = 0
        toBeList = 1   #這層還需要加入result的元素個數
        if root == None:
            return list
        list.append(root)
        while list != []:
            treeNode =
 
 list.pop(0)
            toBeList -= 1
            result.append(treeNode.val)
            if treeNode.left:
                nextLevel += 1
                list.append(treeNode.left)
            if treeNode.right:
                nextLevel += 1
                list.append(treeNode.right)
            if toBeList == 0:
                results.append(result)
                result = []
                toBeList = nextLevel
                nextLevel = 0
        return results

測試：

root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)
root.right.left = TreeNode(5)
root.right.right = TreeNode(6)
results = Solution().PrintFromTopToBottom(root)
for i in results:
    print(i)

結果：

[1]
[2, 3]
[4, 5, 6]