[102] Binary Tree Level Order Traversal

https://leetcode.com/problems/binary-tree-level-order-traversal/description/

• algorithms
• Medium (45.62%)
• Source Code: 102.binary-tree-level-order-traversal.py
• Total Accepted: 359.8K
• Total Submissions: 751.8K
• Testcase Example: '[3,9,20,null,null,15,7]'

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

class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        ret = []
        if not root: return ret
        queue = [root]
        while queue:
            ret.append([i.val for i in queue])
            lenq = len(queue)
            for i in range(lenq):
                if queue[i].left:
                    queue.append(queue[i].left)
                if queue[i].right:
                    queue.append(queue[i].right)
            queue = queue[lenq:]
        return ret