#include <bits/stdc++.h> #define pii pair<int, int> #define db(x) cerr << #x << "=" << x << endl #define db2(x, y) cerr << #x << "=" << x << " , " << #y << "=" << y << endl #define db3(a,b,c) cerr<<#a<<"="<<a<<","<<#b<<"="<<b<<","<<#c<<"="<<c<<endl #include <ext/pb_ds/assoc_container.hpp> #include <ext/pb_ds/tree_policy.hpp> using namespace std; using namespace __gnu_pbds; typedef tree<pair<long long, int>, null_type, less<pair<long long, int>>, rb_tree_tag, tree_order_statistics_node_update> ordered_set; main(){ ordered_set left1, right1; int n; cin >> n; vector<long long> nums; for (int i = 0; i < n; i++){ long long g; cin >> g; nums.push_back(g); right1.insert({g, i}); } long long ans = 0; for (int i = 0; i < n; i++){ right1.erase({nums[i], i}); ans += left1.order_of_key({nums[i], 0LL}) * right1.order_of_key({nums[i], 0LL}); left1.insert({nums[i], i}); } cout << ans << endl; }

Mountains.cpp:11:6: warning: ISO C++ forbids declaration of 'main' with no type [-Wreturn-type]
 main(){
      ^