1001.Sum Problem

PRoblem Description Hey, welcome to HDOJ(Hangzhou Dianzi University Online Judge). In this problem, your task is to calculate SUM(n) = 1 + 2 + 3 + ... + n. Input The input will consist of a series of integers n, one integer per line. Output For each case, output SUM(n) in one line, followed by a blank line. You may assume the result will be in the range of 32-bit signed integer. Sample Input 1 100 Sample Output 1 5050

简单题

#include<iostream> using namespace std; int main() { long long int a; while (cin >> a) { cout << a*(a + 1) / 2 << endl<<endl; } }