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Java Program for Find the number of islands | Set 1 (Using DFS)

  • Last Updated : 30 Dec, 2021

Given a boolean 2D matrix, find the number of islands. A group of connected 1s forms an island. For example, the below matrix contains 5 islands
Example: 
 

Input : mat[][] = {{1, 1, 0, 0, 0},
                   {0, 1, 0, 0, 1},
                   {1, 0, 0, 1, 1},
                   {0, 0, 0, 0, 0},
                   {1, 0, 1, 0, 1} 
Output : 5

This is a variation of the standard problem: “Counting the number of connected components in an undirected graph”. 
Before we go to the problem, let us understand what is a connected component. A connected component of an undirected graph is a subgraph in which every two vertices are connected to each other by a path(s), and which is connected to no other vertices outside the subgraph. 
For example, the graph shown below has three connected components. 
 

 

Java




// Java program to count islands in boolean 2D matrix
import java.util.*;
import java.lang.*;
import java.io.*;
 
class Islands
{
    //No of rows and columns
    static final int ROW = 5, COL = 5;
 
    // A function to check if a given cell (row, col) can
    // be included in DFS
    boolean isSafe(int M[][], int row, int col,
                   boolean visited[][])
    {
        // row number is in range, column number is in range
        // and value is 1 and not yet visited
        return (row >= 0) && (row < ROW) &&
               (col >= 0) && (col < COL) &&
               (M[row][col]==1 && !visited[row][col]);
    }
 
    // A utility function to do DFS for a 2D boolean matrix.
    // It only considers the 8 neighbors as adjacent vertices
    void DFS(int M[][], int row, int col, boolean visited[][])
    {
        // These arrays are used to get row and column numbers
        // of 8 neighbors of a given cell
        int rowNbr[] = new int[] {-1, -1, -10, 01, 1, 1};
        int colNbr[] = new int[] {-101, -1, 1, -1, 0, 1};
 
        // Mark this cell as visited
        visited[row][col] = true;
 
        // Recur for all connected neighbours
        for (int k = 0; k < 8; ++k)
            if (isSafe(M, row + rowNbr[k], col + colNbr[k], visited) )
                DFS(M, row + rowNbr[k], col + colNbr[k], visited);
    }
 
    // The main function that returns count of islands in a given
    //  boolean 2D matrix
    int countIslands(int M[][])
    {
        // Make a bool array to mark visited cells.
        // Initially all cells are unvisited
        boolean visited[][] = new boolean[ROW][COL];
 
 
        // Initialize count as 0 and traverse through the all cells
        // of given matrix
        int count = 0;
        for (int i = 0; i < ROW; ++i)
            for (int j = 0; j < COL; ++j)
                if (M[i][j]==1 && !visited[i][j]) // If a cell with
                {                                 // value 1 is not
                    // visited yet, then new island found, Visit all
                    // cells in this island and increment island count
                    DFS(M, i, j, visited);
                    ++count;
                }
 
        return count;
    }
 
    // Driver method
    public static void main (String[] args) throws java.lang.Exception
    {
        int M[][]=  new int[][] {{1, 1, 0, 0, 0},
                                 {0, 1, 0, 0, 1},
                                 {1, 0, 0, 1, 1},
                                 {0, 0, 0, 0, 0},
                                 {1, 0, 1, 0, 1}
                                };
        Islands I = new Islands();
        System.out.println("Number of islands is: "+ I.countIslands(M));
    }
} //Contributed by Aakash Hasija

Please refer complete article on Find the number of islands | Set 1 (Using DFS) for more details!
 


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