Summation of basic exercise sequence of test questions

Resource constraints

The time limit ：1.0s Memory limit ：256.0MB

Problem description

seek 1+2+3+...+n Value .

Input format

Input includes an integer n.

Output format

Output one line , Include an integer , Express 1+2+3+...+n Value .

The sample input

4

Sample output

10

The sample input

100

Sample output

5050

Data scale and agreement

1 <= n <= 1,000,000,000.

explain ： Notice the size of the data here .

It's easy to get wrong ： The direct idea of this problem is to directly use a cycle to accumulate , However , When the data scale is large , such “ violence ” This method often results in timeouts .

Another point worth noting is that the size of the answer is not in the default integer of your language (int) Within the scope of , If you use integers to save results , It can lead to wrong results . Need to use long long Type integer .

The core idea ：

To use the summation formula of an arithmetic sequence ：

Code ：

#include<iostream>
using namespace std;
int main()
{
	long long int num;
	long long int sum=0;
	cin>>num;
	sum = (1+num)*num/2;
	cout<<sum;
	return 0;
}