Given a binary grid where 1 is land and 0 is water, find the largest island's area.
An island is a group of 1's connected horizontally or vertically. The area is the number of cells.
With grid:
0 0 1 0 0 0 0 1 0 0 0 0 0
0 0 0 0 0 0 0 1 1 1 0 0 0
0 1 1 0 1 0 0 0 0 0 0 0 0
0 1 0 0 1 1 0 0 1 0 1 0 0
0 1 0 0 1 1 0 0 1 1 1 0 0
0 0 0 0 0 0 0 0 0 0 1 0 0
0 0 0 0 0 0 0 1 1 1 0 0 0
0 0 0 0 0 0 0 1 1 0 0 0 0
The largest island has area 6.
Constraints: Grid up to .