Find Number of Islands in 2D Matrix?

int NumIslands(char[][] grid)

if (grid == null || grid.Length == 0) return 0;

int count = 0;

for (int i = 0; i < grid.Length; i++)

for (int j = 0; j < grid[0].Length; j++)

if (grid[i][j] == '1')

Follow on:

DFS(grid, i, j);

count++;

return count;

void DFS(char[][] grid, int i, int j)

if (i < 0 || j < 0 || i >= grid.Length || j >= grid[0].Length ||

grid[i][j] == '0')

return;

grid[i][j] = '0'; // Mark visited

DFS(grid, i + 1, j);

DFS(grid, i - 1, j);

DFS(grid, i, j + 1);

DFS(grid, i, j - 1);

Explanation:

Use DFS to mark all connected land cells, count islands by visiting unvisited lands.