102.md

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]
]

my thoughts:

1. bfs.
O(n)

my solution:

**********48ms
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        res = []
        if not root:
            return res
        prev = [root]
        while prev:
            cur = []
            curVal = []
            for n in prev:
                curVal.append(n.val)
                if n.left:
                    cur.append(n.left)
                if n.right:
                    cur.append(n.right)
            res.append(curVal)
            prev = cur
        return res

my comments:

from other ppl's solution:

1. N/A