LintCode "Number of Islands II"

A typical Union-Find one. I'm using a kinda Union-Find solution here. Some boiler-plate code - yeah I know.

class Solution {
    unordered_set<int> hs; // start from 1
    int max_k;
public:
    void dfs(vector<vector<int>> &mt, int x, int y, int f)
    {
    int n = mt.size(), m = mt[0].size();
    mt[y][x] = f;
    if(x > 0 && mt[y][x - 1] != f && mt[y][x - 1] != 0) // left
    {
        mt[y][x - 1] = f;
        dfs(mt, x - 1, y, f);
    }
    if(x < m - 1 && mt[y][x + 1] != f && mt[y][x + 1] != 0) // right
    {
        mt[y][x + 1] = f;
        dfs(mt, x + 1, y, f);
    }
    if(y > 0 && mt[y - 1][x] != f && mt[y - 1][x] != 0) // up
    {
        mt[y - 1][x] = f;
        dfs(mt, x, y - 1, f);
    }
    if(y < n - 1 && mt[y + 1][x] != f && mt[y + 1][x] != 0) // down
    {
        mt[y + 1][x] = f;
        dfs(mt, x, y + 1, f);
    }
    }
    void update(vector<vector<int>> &mt, Point &p)
    {
    int n = mt.size(), m = mt[0].size();
    
    int minf = INT_MAX;
    vector<Point> to_union;
    if(p.x > 0) // left
    {
        if(mt[p.y][p.x - 1] != 0)
        {
        minf = min(minf, mt[p.y][p.x - 1]);
        to_union.push_back(Point(p.x - 1, p.y));
        }
    }
    if(p.x < m - 1) // right
    {
        if(mt[p.y][p.x + 1] != 0)
        {
        minf = min(minf, mt[p.y][p.x + 1]);
        to_union.push_back(Point(p.x + 1, p.y));
        }
    }
    if(p.y > 0) // update
    {
        if(mt[p.y - 1][p.x] != 0)
        {
        minf = min(minf, mt[p.y - 1][p.x]);
        to_union.push_back(Point(p.x, p.y - 1));
        }
    }
    if(p.y < n - 1) // down
    {
        if(mt[p.y + 1][p.x] != 0)
        {
        minf = min(minf, mt[p.y + 1][p.x]);
        to_union.push_back(Point(p.x, p.y + 1));
        }
    }
    ////
    if(minf == INT_MAX)
    {
        int np = max_k ++;        
        mt[p.y][p.x] = np;  
        hs.insert(np);
    }
    else
    {
        mt[p.y][p.x] = minf;
        for(auto &tp:to_union)
        {
            if(mt[tp.y][tp.x] != 0 && mt[tp.y][tp.x] != minf)
            {
                int old = mt[tp.y][tp.x];
                dfs(mt, tp.x, tp.y, minf);
                hs.erase(old);            
            }
        }
    }
    }
    /**
     * @param n an integer
     * @param m an integer
     * @param operators an array of point
     * @return an integer array
     */
    vector<int> numIslands2(int n, int m, vector<Point>& operators){
        vector<vector<int>> mt(m, vector<int>(n));
        max_k = 1;
        vector<int> ret;
        for(auto &p : operators)
        {
            update(mt, p);
            ret.push_back(hs.size());
        }
        return ret;
    }
};