for Robot Artificial Inteligence

59. number of islands [어려움]

30 Jul 2020 | Daily Algorithms

  • I saw many people post DFS solutions but fewer BFS ones. So I wrote one below. Each time when I see a ‘1’, I increment the counter and then erase all connected ‘1’s using a queue.

BFS methods

class Solution {
public:
    int numIslands(vector<vector<char>>& grid) {
        int m = grid.size(); // row
        int n = m ? grid[0].size() : 0; // column
        int islands = 0;
        int offsets[] = {0,1,0,-1,0};
        for(int i =0; i<m;i++)
        {
            for(int j=0; j<n; j++)
            {
                if(grid[i][j]=='1')
                {
                    islands++;
                    grid[i][j] = '0';
                    queue<pair<int,int>> todo;
                    todo.push({i,j});
                    while(!todo.empty())
                    {
                        pair<int,int> p = todo.front();
                        todo.pop();
                        for(int k =0; k<4; k++)
                        {
                            int r = p.first + offsets[k];
                            int c = p.second + offsets[k+1];
                            if(r>=0 && r<m && c>=0 && c<n && grid[r][c]=='1')
                            {
                                grid[r][c] = '0';
                                todo.push({r,c});
                            }
                        }
                    }

                }
            }
        }
        return islands;
    }
};

DFS

  • Or I can erase all the connected ‘1’s using DFS
class Solution {
public:
    int numIslands(vector<vector<char>>& grid) {
        int m = grid.size(), n = m ? grid[0].size() : 0, islands = 0;
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (grid[i][j] == '1') {
                    islands++;
                    eraseIslands(grid, i, j);
                }
            }
        }
        return islands;
    }
private:
    void eraseIslands(vector<vector<char>>& grid, int i, int j) {
        int m = grid.size(), n = grid[0].size();
        if (i < 0 || i == m || j < 0 || j == n || grid[i][j] == '0') {
            return;
        }
        grid[i][j] = '0';
        eraseIslands(grid, i - 1, j);
        eraseIslands(grid, i + 1, j);
        eraseIslands(grid, i, j - 1);
        eraseIslands(grid, i, j + 1);
    }
};

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