3 VOs:
1. coding: number of islands. Given a matrix, find the number of islands, follow up: how to optimize if the m--len(matrix)-- or rows, is large.
2. coding: Evaluate math expression via binary tree: "(3+4)*2". Task: evaluate the expression
3. coding: System design. Robot collecting items. Design class objects, how to do each task: locate, collect, and return the item given locations
