# Find the number of islands Using DFS in C++

C++Server Side ProgrammingProgramming

In this problem, we are given a 2D binary matrix. Our task is to Find the number of islands Using DFS.

Island is a ground of 1 or more connected 1’s in the matrix.

Let’s take an example to understand the problem,

Input : bin[][] = {{ 1 0 0 0}
{0 1 0 1}
{0 0 0 0}
{0 0 1 0}}
Output : 3
ExplanationIslands are −bin00 - bin11bin13bin32


## Solution Approach

To solve the problem using DFS, we will use the DFS technique for exploring all the neighbours (maximum possible 8 of a number in the matrix) and check for 1’s. If we encounter 1 value which is unvisited, then we will consider it. We will keep a check on the values that are visited to avoid repeated visits. Doing so, we can count the number of islands present in the matrix.

Learn DFS on graphs

## Example

#include <bits/stdc++.h>
using namespace std;
#define ROW 5
#define COL 5
int canVisit(int bin[][COL], int row, int col, bool visited[][COL]){
return (row >= 0) && (row < ROW) && (col >= 0) && (col < COL) && (bin[row][col] && !visited[row][col]);
}
void DFS(int bin[][COL], int row, int col, bool visited[][COL]){
static int getNeighbourRow[] = { -1, -1, -1, 0, 0, 1, 1, 1 };
static int getNeighbourCol[] = { -1, 0, 1, -1, 1, -1, 0, 1 };
visited[row][col] = true;
for (int k = 0; k < 8; ++k)
if (canVisit(bin, row + getNeighbourRow[k], col + getNeighbourCol[k], visited))
DFS(bin, row + getNeighbourRow[k], col + getNeighbourCol[k], visited);
}
int findIslandCount(int bin[][COL]){
bool visited[ROW][COL];
memset(visited, 0, sizeof(visited));
int islandCount = 0;
for (int i = 0; i < ROW; ++i)
for (int j = 0; j < COL; ++j)
if (bin[i][j] && !visited[i][j]) {
DFS(bin, i, j, visited);
islandCount++;
}
return islandCount;
}
int main(){
int bin[][COL] = {{1, 0, 0, 0},
{0, 1, 0, 1},
{0, 0, 0, 0},
{0, 0, 1, 0}};
cout<<"The number of islands present in the matrix is "<<findIslandCount(bin);
return 0;
}

## Output

The number of islands present in the matrix is 4
Updated on 11-Feb-2022 10:27:51