Count Unreachable Pairs of Nodes in an Undirected Graph.cpp

class Solution {
public:
    typedef long long ll;
    void dfs(int node, unordered_map<int,vector<int>>& m, ll& cnt, vector<int>& vis){
        vis[node] = 1;
        cnt++;
        for(auto& i: m[node]){
            if(vis[i]==0) dfs(i,m,cnt,vis);   
        }
    }
    long long countPairs(int n, vector<vector<int>>& edges) {
        unordered_map<int,vector<int>> m; // making adjacency list
        for(int i=0;i<edges.size();i++){
            m[edges[i][0]].push_back(edges[i][1]);
            m[edges[i][1]].push_back(edges[i][0]);
        }
        ll ans = ((ll)n*(n-1))/2;
        vector<int> vis(n,0);
        for(int i=0;i<n;i++){
            if(vis[i]==0){ // as node is not visited, we find the no. of nodes in current component.
                ll cnt = 0;
                dfs(i,m,cnt,vis);
                ans -= (cnt*(cnt-1))/2;
            }
        }
        return ans;
    }
};