Number of Islands

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

Answer: 3

Analyse: BFS or DFS. If found a '1', label that element as visited, and find all elements could be reached from that element and label them. 

Runtime: Iteration 12ms

class Solution {
public:
    int numIslands(vector<vector<char>>& grid) {
        if(grid.empty() || grid[0].empty()) return 0;
        
        int result = 0;
        for(int i = 0; i < grid.size(); i++) {
            for(int j = 0; j < grid[i].size(); j++) {
                if(grid[i][j] == '1') {
                    grid[i][j] = '2';
                    bfs(grid, i, j);
                    result++;
                }
            }
        }
        return result;
    }
    
    void bfs(vector<vector<char> >& grid, int i, int j) {
        queue<pair<int, int> > qu;
        qu.push(make_pair(i, j));
        
        int m = grid.size(), n = grid[0].size();
        while(!qu.empty()) {
            pair<int, int> node = qu.front();
            i = node.first;
            j = node.second;
            qu.pop();
            // check whether its neighbour is island
            if(i > 0 && grid[i - 1][j] == '') {
                qu.push(make_pair(i - 1, j));
                grid[i - 1][j] = '2';
            }
            if(j > 0 && grid[i][j - 1] == '1') {
                qu.push(make_pair(i, j - 1));
                grid[i][j - 1] = '2';
            }
            if(j < n - 1 && grid[i][j + 1] == '1') {
                qu.push(make_pair(i, j + 1));
                grid[i][j + 1] = '2';
            }
            if(i < m - 1 && grid[i + 1][j] == '1') {
                qu.push(make_pair(i + 1, j));
                grid[i + 1][j] = '2';
            }
        }
    }
};

Runtime: Recursive 8ms.

class Solution {
public:
    int numIslands(vector<vector<char>>& grid) {
        int result;
        if(grid.empty() || grid[0].empty()) return result;
        
        int row = grid.size(), col = grid[0].size();
        for(int i = 0; i < row; i++){
            for(int j = 0; j < col; j++){
                if(grid[i][j] == '1'){
                    helper(grid, i, j);
                    result++;
                }
            }
        }
        return result;
    }
    
    void helper(vector<vector<char> > &grid, int x, int y){
        if(x >= grid.size() || x < 0 || y >= grid[0].size() || y < 0 || grid[x][y] != '1')
            return;
            
        grid[x][y] = '2';
        helper(grid, x + 1, y);
        helper(grid, x - 1, y);
        helper(grid, x, y + 1);
        helper(grid, x, y - 1);
    }
};