从零开始刷HDOJ(2)【HDOJ1001 - Sum Problem】
2017-05-19 16:45
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从零开始刷HDOJ(2)【HDOJ1001 - Sum Problem】
题面
Sum ProblemTime 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; }
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