G5 - Number of Islands

May 21, 2017

Given a 2d grid map of '1's (land) and '0's (water), count the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.
Example 1:
11110
11010
11000
00000
Answer: 1
Example 2:
11000
11000
00100
00011

Solution :
pick one point and do dfs and mark visited all the points connected to it.

                
public class Solution {
    public int numIslands(char[][] grid) {
        int count = 0;
        int row = grid.length;
        if(row == 0)
            return 0;
        int col = grid[0].length;
        boolean[][] visited = new boolean[row][col];
        for(int i=0; i<row; i++){
            for(int j=0; j<col; j++){
                if(grid[i][j] == '1' && visited[i][j] == false){
                    dfs(grid, i, j, row, col, visited);
                    count++;
                }
            }
        }
        return count;
    }

    public void dfs(char[][] grid, int i, int j, int row, int col, boolean[][] visited){
        if(i < 0 || i >= row || j < 0 || j>= col){
            return;
        }else if(grid[i][j] == '1' && visited[i][j] == false){
            visited[i][j] = true;
            dfs(grid, i+1, j, row, col, visited);
            dfs(grid, i-1, j, row, col, visited);
            dfs(grid, i, j+1, row, col, visited);
            dfs(grid, i, j-1, row, col, visited);
        }
    }
}