#include <iostream>
using namespace std;
#include<unordered_map>
int pairSum(int *arr, int n) {
	// Write your code here
    //create an unordered_ map to store the frequency of a number in it
    unordered_map<int,int> freq;
    for(int i = 0;i<n;i++){
        freq[arr[i]]++;
    }
    //we will the find the pair of an element
    //for every element if freq = n and n pair is present we will print them n*n times coz every element has n optuions to go to
    int count = 0;
    for(int i = 0;i<n;i++){
        if(freq.find(arr[i])!=freq.end() && freq.find(-arr[i])!=freq.end()){
        if(freq[arr[i]] == freq[-arr[i]]){
            count = count + 2*freq[arr[i]];
          
            freq.erase(arr[i]);
          
        }
        }
        if(freq.find(arr[i])!=freq.end() && freq.find(-arr[i])!=freq.end()){
          if(freq[arr[i]]>freq[-arr[i]]){
              count = count + freq[arr[i]];
          
              freq.erase(arr[i]);
          }
        }
    }
    return count;
}
int main() {
    int n;
    cin >> n;

    int* arr = new int[n];

    for (int i = 0; i < n; ++i) {
        cin >> arr[i];
    }

    cout << pairSum(arr, n);

    delete[] arr;
}
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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
}