Can i reduce memory use in this c++ code?

Question

#include <bits/stdc++.h>
using namespace std;

int n;

vector<bool> used;
 vector<int> order, comp;

void dfs1 (int v,const vector<vector<int>>& g) {
used[v] = true;
for (size_t i=0; i<g[v].size(); ++i) {
    int to = g[v][i];
    if (!used[to])
        dfs1 (to,g);
}
order.push_back (v);
}

void dfs2 (int v, int cl,const vector<vector<int>>& gt) {
comp[v] = cl;
for (size_t i=0; i<gt[v].size(); ++i) {
    int to = gt[v][i];
    if (comp[to] == -1)
        dfs2 (to, cl,gt);
}
}

  int main() {
  int q1,q3;
  cin>>q1>>n>>q3;

  n*=2;
  vector < vector<int> > g(n), gt(n);
  vector< vector< int> > adj(q1+5);
    int o,oo;
  for(int i=0;i<q3;i++){
         cin>>o>>oo;
         if(oo<0){
           oo+=1;
           adj[o].push_back((oo*-1)*2+1);   
         } else{

           oo-=1;
              adj[o].push_back(oo*2);

         }

  }   

  for(int i=1;i<=q1;i++){

       for(int j=0;j<adj[i].size();j++){
            for(int k=0;k<adj[i].size();k++){

                    if(j!=k){
                   g[adj[i][j]^1].push_back(adj[i][k]);
                        gt[adj[i][k]].push_back(adj[i][j]^1);
                       }
              } }
              }  



     used.assign (n+1, false);
 for (int i=0; i<n; ++i)
    if (!used[i])
        dfs1 (i,g);

comp.assign (n+1, -1);
for (int i=0, j=0; i<n; ++i) {
    int v = order[n-i-1];
    if (comp[v] == -1)
        dfs2 (v, j++,gt);
}

for (int i=0; i<n; ++i)
    if (comp[i] == comp[i^1]) {
 cout<<-1;
        return 0;
    }
vector<int> answ;

for (int i=0; i<n; i+=2) {
    int ans = comp[i] > comp[i^1] ? i : i^1;
      if(ans%2==0){
        ans/=2;

        ans++;
              answ.push_back(ans);

  }
}
cout<<answ.size()<<endl;
for(int i=0;i<answ.size();i++){
cout<<answ[i]<<endl;
}



 }

This is code which solves problem connected with 2SAT. When entering value n=200 000 this code exceeds memory limit which is 512 mB. What tricks i can use to reduice memory use? I have already tried assigning vectors g and gt in main code, and then in dfs and dfs2 functions I put g and gt but that gave me time limit issue. How i can optimise this code?

Answer 1

You are not using g and gt at the same time. So clear g and reserve gt after that. ( comp.assign (n+1, -1); before this line )

Same with used and comp.

used and adj are not used at the same time. Clear adj before line used.assign (n+1, false);

Try using adj as a C style vector( int** ) and another vector for the individual sizes.