OA: 比较简单的题目，主要考BFS, DFS，题目定期更换目
第一轮：白人面试官，题目是Diameter of Binary Tree
第二轮：白人manager+亚洲人senior，考的system design，题目是design a system that can retrieve and upload pictures related to products
第三轮：黑人面试官，题目是设计in-memory key-value store
第四轮：中国人manager+中国人sde，题目是word search II
Tell me a time when you encountered unexpected obstacles
Tell me a time when you disagree with your boss? Follow up: When you strongly disagree with your boss, what would you do?
Tell me when you have to meet tight deadline