Given an nxn grid of 1s and 0s, return the number of islands in the input.

To count the number of islands in a 2D grid, we can iterate through each cell. If a cell contains a '1' (representing land) and hasn't been visited yet, we increment our island count. Then, we initiate a Depth First Search (DFS) or Breadth First Search (BFS) from that cell to explore and mark all connected land cells as visited. This ensures that each connected component of land is counted as a single island. The DFS/BFS effectively 'sinks' the island by marking its cells, preventing recounting.