102. Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / 
  9  20
    /  
   15   7

return its level order traversal as:

[
  [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
#递归解法
class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if root == None:
            return []
        node_list = []
        node_list.append(root)
        list_value = []
        self.helper(node_list, list_value)
        return list_value 
    def helper(self, node_list, list_value):
        tmp_l_value = []
        tmp_l_node = []
        if node_list != []:
            for i in node_list:
                tmp_l_value.append(i.val)
                if i.left != None:
                    tmp_l_node.append(i.left)
                if i.right != None:
                    tmp_l_node.append(i.right)
            list_value.append(tmp_l_value)
            self.helper(tmp_l_node, list_value)
        
        
    
        

 #非递归，用队列解法

# 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]]
        """
        if not root :
            return []
        from collections import deque
        q_node = deque([root])
        list_value = []
        while q_node:
            tmp_value = []
            size = len(q_node)
            for i in range(size):
                node = q_node.popleft()
                if node.left:
                    q_node.append(node.left)
                if node.right:
                    q_node.append(node.right)
                tmp_value.append(node.val)
            list_value.append(tmp_value)
        return list_value