0947.cpp

class Solution {
  public:
    int removeStones(vector<vector<int>> &stones) {
        unordered_map<int, vector<int>> rows, cols;
        unordered_set<int> seen;

        for (int s = 0; s < size(stones); ++s)
            rows[stones[s][0]].push_back(s), cols[stones[s][1]].push_back(s);

        int c = 0;
        stack<int> st;
        for (int s = 0; s < size(stones); ++s) {
            if (seen.count(s)) continue;
            st.push(s);
            while (!st.empty()) {
                int s = st.top();
                st.pop();
                if (seen.count(s)) continue;
                seen.insert(s);
                int r = stones[s][0], c = stones[s][1];
                for (auto ss : rows[r])
                    if (!seen.count(ss)) st.push(ss);
                for (auto ss : cols[c])
                    if (!seen.count(ss)) st.push(ss);
            }
            c++;
        }

        return size(stones) - c;
    }
};