Islands.java

/*
In a 2d matrix, there are islands are surrounded by water. 
For easier understand, we use 1 stands for islands, 0 stands for water.
A example would be: 
1 0 0 0
1 1 1 0
0 0 1 0
1 0 0 0

There are 2 islands in the matrix. 
the first one is:
1
1 1 1
    1
    
The second one is 
1 (on the index of( 3, 0) 

Find numbers of islands by giving a 2d matrix. 
*/
class Island {
  public static void main(String[] args) {
    int[][] matrix = {{1, 0, 0, 0},
                      {1, 1, 1, 0},
                      {0, 0, 1, 0},
                      {1, 0, 0, 0}};
    System.out.println(numOfIslands(matrix));
    // here should print 2
    
     int[][] matrix2 = {{1, 0, 0, 0},
                       {1, 0, 1, 0},
                       {0, 0, 1, 0},
                       {1, 0, 0, 0}};
    System.out.println(numOfIslands(matrix2));
    // here should print 3
  }
  private int numOfIslands(int[][] matrix) {

      int countIslands = 0;
      for(int i = 0; i < matrix.length; i++) {
         for(int j = 0; j < matrix[0].length; j++) {
             if(matrix[i][j] == 1) {
                DFS(matrix, i, j);
                countIslands++;
             }
         }
      }
      return countIslands;
  }
  private void DFS(int[][] matrix, int row, int col) {

    if(row < 0 || row >= matrix.length || col < 0 || col > matrix[0].length || matrix[row][col] != 1) {
       return; 
    }
    matrix[row][col] = 2;
    DFS(matrix, row+1, col);
    DFS(matrix, row-1, col);
    DFS(matrix, row, col+1);
    DFS(matrix, row, col-1);
  }
}