#include<bits/stdc++.h>
#define FOR(x,a,b) for(int x=a;x<=b;x++)
#define FOD(x,a,b) for(int x=a;x>=b;x--)
#define pub push_back
#define pob pop_back
#define fi first
#define se second
#define sz(x) x.size()
#define cout(x) cout<<fixed<<setprecision(x)
#define file freopen("h.inp","r",stdin);freopen("h.out","w",stdout);
#define fast ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
typedef unsigned long long ull;
typedef long long ll;
const int N=1e6+5;
const int mod=2021;
using namespace std;
ll n,a[N];
void solve(){
    map<int,int>mp;
    ll h=0;
    FOR(i,1,n){cin>>a[i];mp[a[i]]++;}
    FOR(i,1,n)if(a[i]>0&&mp[a[i]*-1]){
        h+=mp[a[i]*-1]*mp[a[i]];
        mp[a[i]]=0;
        mp[-a[i]]=0;
    }
    h+=mp[0]*(mp[0]-1)/2;
    cout<<h<<endl;
}
int main(){
    fast
    //file
    cin>>n;
    solve();
    return 0;
}
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C++ Online Compiler

Write, Run & Share C++ code online using OneCompiler's C++ online compiler for free. It's one of the robust, feature-rich online compilers for C++ language, running on the latest version 17. Getting started with the OneCompiler's C++ compiler is simple and pretty fast. The editor shows sample boilerplate code when you choose language as C++ and start coding!

Read inputs from stdin

OneCompiler's C++ online compiler supports stdin and users can give inputs to programs using the STDIN textbox under the I/O tab. Following is a sample program which takes name as input and print your name with hello.

#include <iostream>
#include <string>
using namespace std;

int main() 
{
    string name;
    cout << "Enter name:";
    getline (cin, name);
    cout << "Hello " << name;
    return 0;
}

About C++

C++ is a widely used middle-level programming language.

  • Supports different platforms like Windows, various Linux flavours, MacOS etc
  • C++ supports OOPS concepts like Inheritance, Polymorphism, Encapsulation and Abstraction.
  • Case-sensitive
  • C++ is a compiler based language
  • C++ supports structured programming language
  • C++ provides alot of inbuilt functions and also supports dynamic memory allocation.
  • Like C, C++ also allows you to play with memory using Pointers.

Syntax help

Loops

1. If-Else:

When ever you want to perform a set of operations based on a condition If-Else is used.

if(conditional-expression) {
   //code
}
else {
   //code
}

You can also use if-else for nested Ifs and If-Else-If ladder when multiple conditions are to be performed on a single variable.

2. Switch:

Switch is an alternative to If-Else-If ladder.

switch(conditional-expression){    
case value1:    
 // code    
 break;  // optional  
case value2:    
 // code    
 break;  // optional  
......    
    
default:     
 code to be executed when all the above cases are not matched;    
}

3. For:

For loop is used to iterate a set of statements based on a condition.

for(Initialization; Condition; Increment/decrement){  
  //code  
}

4. While:

While is also used to iterate a set of statements based on a condition. Usually while is preferred when number of iterations are not known in advance.

while (condition) {  
// code 
}

5. Do-While:

Do-while is also used to iterate a set of statements based on a condition. It is mostly used when you need to execute the statements atleast once.

do {  
 // code 
} while (condition);

Functions

Function is a sub-routine which contains set of statements. Usually functions are written when multiple calls are required to same set of statements which increases re-usuability and modularity. Function gets run only when it is called.

How to declare a Function:

return_type function_name(parameters);

How to call a Function:

function_name (parameters)

How to define a Function:

return_type function_name(parameters) {  
 // code
}