Number of Islands problem.

Solution.java

package com.interviewbytes.islands;

public class Solution {

    public int numIslands(char[][] grid) {

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

        final int M = grid.length;
        final int N = grid[0].length;

        boolean visited[][] = new boolean[M][N];

        int islandCount = 0;

        for (int i = 0; i < M; i++) {
            for (int j = 0; j < N; j++) {
                if (!visited[i][j] && grid[i][j] == '1') {
                    dfs(grid, visited, i, j, M, N);
                    islandCount++;
                }
            }
        }

        return islandCount;
    }

    private void dfs(char[][] grid, boolean[][] visited, int i, int j, int m, int n) {

        if (!isValid(i, j, m, n) || grid[i][j] != '1' || visited[i][j]) return;

        visited[i][j] = true;

        dfs(grid, visited, i - 1, j, m, n);
        dfs(grid, visited, i + 1, j, m, n);
        dfs(grid, visited, i, j - 1, m, n);
        dfs(grid, visited, i, j + 1, m, n);
    }


    private boolean isValid(int i, int j, int m, int n) {
        return i >= 0 && i < m && j >= 0 && j < n;
    }
}