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]
]

解题思路：该题的目的是做层序遍历，使用宽度优先搜索（BST），用一个队列queue保存一层的结点，通过遍历queue，取得该层结点的value值，并且在new_q里同步保存下一层结点

class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if not root:
            return []
        queue = [root]
        result = []
        while(queue):
            level = []
            new_q = []
            for i in queue:
                node = i
                #保存每一层结点的value
                level.append(node.val)
                #将下一层的结点保存在new_q队列，当queue队列遍历结束后赋值给queue
                if(node.left):
                    new_q.append(node.left)
                if(node.right):
                    new_q.append(node.right)
            queue = new_q    
            result.append(level)  
        return result