grid is a map with m rows and n columns: 1 is land and 0 is water. Land cells that touch along a side (up, down, left or right) belong to the same island; cells that meet only at a corner do not. Everything outside the grid is water.
An island's area is the number of land cells in it. Return the area of the largest island, or 0 if the grid has no land.
There are four islands, of areas 3 (top left), 3 (top right), 4 (bottom left) and 1 (bottom right). The top-right island and the bottom-left one meet only at a corner, so they stay separate rather than forming an island of 7.
The ring is one island. The water in the middle is not part of it.
Constraints
m == grid.lengthn == grid[i].length1 <= m, n <= 50grid[i][j] is 0 or 1.Scan the grid. Each time you reach land you haven't visited, you've found a new island: explore all of it and count its cells.
4