从零开始刷HDOJ(2)【HDOJ1001 – Sum Problem】
题面
Sum Problem
Time Limit: 1000/500 MS (Java/Others) Memory Limit: 65536/32768 K (Java/Others)
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
Author
DOOM III
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翻译
每行输入一个数,然后隔一行输出一个数,这个数是从1加到输入的那个数的和。
思路
循环?肯定是不行啊,因为每一行的输入都是32位的。所以,应该用到一个众所周知的公式——
∑i=1ni=n(n+1)2
所以,每一个数都可以O(1)求了。
代码
#include <iostream>
int main(int argc, char ** argv)
{
std::ios_base::sync_with_stdio(false);
long long x;
while (std::cin >> x)
std::cout << x * (x + 1) / 2 << std::endl << std::endl;
//底下就是暂停用的,不用管。
#ifdef __EDWARD_EDIT
std::cin.get();
std::cin.get();
#endif
return 0;
}