Given an mxn 2D binary grid which represents a map of 1s (land) and 0s (water), return the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.

number_of_islands.java

class Solution {
    public int numIslands(char[][] grid) {
        int count = 0; // initialize the count to 0
        if (grid.length == 0) { // if the grid is empty, return 0
            return 0;
        }
        // iterate through each cell in the grid
        for (int i = 0; i < grid.length; i++) {
            for (int j = 0; j < grid[i].length; j++) {
                // if a land cell (1) is found, increment the count
                if (grid[i][j] == '1') {
                    count++;
                    // perform breadth-first search to mark all connected land cells
                    callBFS(grid, i, j);
                }
            }
        }
        return count; // return the total number of islands
    }
    
    // helper method to perform breadth-first search and mark all connected land cells
    public void callBFS(char[][] grid, int i, int j) {
        // base case: if out of bounds or cell is water (0), then return
        if (i < 0 || i >= grid.length || j < 0 || j >= grid[i].length || grid[i][j] == '0') {
            return;
        }
        // mark the current cell as visited by setting it to '0'
        grid[i][j] = '0';
        callBFS(grid, i + 1, j); // down
        callBFS(grid, i - 1, j); // up
        callBFS(grid, i, j + 1); // right
        callBFS(grid, i, j - 1); // left
    }
}