HackerEarth Noor and his pond problem solution

In this HackerEarth Noor and his pond problem solution Noor is going fish farming. There are N types of fish. Each type of fish has size(S) and eating factor(E). A fish with eating factor of E, will eat all the fish of size .

Help Noor to select a set of fish such that the size of the set is maximized as well as they do not eat each other.

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

string toZero(int n)
{
    string ret;
    for(int i = 0; i < 2;i ++) ret.push_back(n%10 + '0'), n/=10;
    reverse(ret.begin(), ret.end());
    return ret;
}

int main()
{    
        int t;
        cin >>t;
        assert(t >= 1 && t <= 3);
        while(t--){
            int n;
            cin >> n;
            assert(n >= 1 && n <= 100000);
            
            vector< pair<int,int> > vec;
            for(int i = 0; i < n; i ++ ){
                int s,e;
                cin >> s >> e;
                assert(s >= 1 && s <= 1000000000);
                assert(e >= 1 && e <= 1000000000);
                assert(s > e);
                vec.push_back( make_pair(e,1));
                vec.push_back( make_pair(s,-1));
            }
            sort(vec.begin(), vec.end());
            int ans = 0, cum = 0;
            for(int i = 0; i < vec.size(); i ++){
                cum += vec[i].second;
                ans = max(ans , cum);
            }
            cout << ans << endl;
        }
    }
    
//}