士兵买香蕉

题目大意：

A soldier wants to buy w bananas in the shop. He has to pay k dollars for the first banana, 2k dollars for the second one and so on (in other words, he has to pay i·k dollars for the i-th banana).

He has n dollars. How many dollars does he have to borrow from his friend soldier to buy w bananas?

输入：

输入三个数，N,K,W，1<=K,W<=1000,0<=N<=10^9，N是第一个香蕉的费用，K是士兵拥有的money,W是士兵要买的香蕉个数。

输出：

士兵需要向朋友借多少（q）钱才能买到自己想买的香蕉

题目分析：

1、依次算出所有香蕉需要花多少钱sum；

2、用sum跟K比较（两种情况：K大于等于sum不用借，否则要借）

3、q=K-sum；

源代码：

 1 #include<iostream>
 2 using namespace std;
 3 int main()
 4 {
 5     int k, n, w, sum = 0, q;
 6     cin >> k >> n >> w;
 7     for (int i = 1; i <= w; i++)
 8         sum += i*k;
 9     if (n == sum||n>sum)
10         cout << "0" << endl;
11     else
12     {
13         q = sum - n;
14         cout << q << endl;
15     }
16
17     return 0;
18
19 }

心得：题目比较简单，但各种情况要考虑到，写代码时最开始就只想到要借钱的那一部分，没有想到还有不借的。犯的错误比较低级。。。。。继续加油吧！O(∩_∩)O~~~