https://leetcode.com/problems/number-of-islands/

answer.cc

class Solution {
public:
    int numIslands(vector<vector<char>>& grid) {
        if (grid.empty() || grid[0].empty()) {
            return 0;
        }
        vector<vector<bool>> checked(grid.size(), vector<bool>(grid[0].size()));
        int islands = 0;
        for (int y = 0; y < grid.size(); y++) {
            for (int x = 0; x < grid[0].size(); x++) {
                if (findNewIsland(grid, &checked, x, y)) {
                    islands++;
                }
            }
        }
        return islands;
    }
    
    bool findNewIsland(const vector<vector<char>>& grid,
               vector<vector<bool>>* checked, int x, int y) {
        if (grid[y][x] == '0' || (*checked)[y][x]) {
            return false;
        }
        (*checked)[y][x] = true;
        if (y < grid.size() - 1 && grid[y+1][x] == '1')
            findNewIsland(grid, checked, x, y + 1);
        if (y > 0 && grid[y-1][x] == '1')
            findNewIsland(grid, checked, x, y - 1);
        if (x < grid[0].size() - 1 && grid[y][x+1] == '1')
            findNewIsland(grid, checked, x + 1, y);
        if (x > 0 && grid[y][x-1] == '1')
            findNewIsland(grid, checked, x - 1, y);
        return true;
    }
};