Count the islands in Java
Java interview exercise on Algorithms & recursion, difficulty 3 of 3. Solve it in the editor beside a worked example, or cold in Interview mode with the example hidden; the tests run the moment you do.
Write countIslands(grid) for a grid of '1' (land) and '0' (water): an island is a group of land cells connected horizontally or vertically. Scan every cell; when you find unvisited land, count one island and flood-fill its cells (recursive DFS or a queue) so they are not counted again. The grid may be modified.
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Interview mode hides the example: solve it from memory, without the hint or the solution, and it counts as solved cold. Progress is saved in your browser, and in your account if you sign in.