102. Binary Tree Level Order Traversal
[https://leetcode.com/problems/binary-tree-level-order-traversal/]
LintCode-69.Binary Tree Level Order Traversal
[http://www.lintcode.com/en/problem/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