class Solution {
    public int numIslands(char[][] grid) {
        int num = 0;

        for (int i=0; i<grid.length;i++)
          for (int j=0;j<grid[i].length;j++)
              if (grid[i][j] == '1'){
                  num++;
                  mark(i,j,grid);
              }

        return num;
    }

    private void mark(int i, int j, char[][] grid) {
        grid[i][j]=0;
        if (i > 0 && grid[i-1][j]=='1') mark(i-1,j,grid);
        if (i < grid.length-1 && grid[i+1][j]=='1') mark(i+1,j,grid);
        if (j > 0 && grid[i][j-1]=='1') mark(i,j-1,grid);
        if (j < grid[i].length-1 && grid[i][j+1]=='1') mark(i,j+1,grid);
    }
}