NumberOfIslandsDFS.java

private final int[] xDirection = {1, 0, -1, 0};
private final int[] yDirection = {0, -1, 0, 1 };

public int numIslands(char[][] grid) {
    if (grid == null || grid.length == 0 || grid[0].length == 0) {
        return 0;
    }

    int m = grid.length;
    int n = grid[0].length;
    int islandCount = 0;
    boolean[][] visited = new boolean[m][n];

    for (int i = 0; i < m; i++) {
        for (int j = 0; j < n; j++) {
            if (grid[i][j] == '0' || visited[i][j]) {
                continue;
            }
            explore(i, j, m, n, grid, visited);
            islandCount++;
        }
    }

    return islandCount;
}

private void explore(int x, int y, int row, int col, char[][] grid, boolean[][] visited) {
    if (!this.shouldExplore(x, y, row, col, grid, visited)) return;

    visited[x][y] = true;
    for (int i = 0; i < 4; i++) {
        explore(x + this.xDirection[i], y + this.yDirection[i], row, col, grid, visited);
    }
}