PTA-1001——A+B Format

题目：

Calculate a+b and output the sum in standard format -- that is, the digits must be separated into groups of three by commas (unless there are less than four digits).

Input Specification:

Each input file contains one test case. Each case contains a pair of integers a and b where −. The numbers are separated by a space.

Output Specification:

For each test case, you should output the sum of a and b in one line. The sum must be written in the standard format.

Sample Input:

-1000000 9

Sample Output:

-999,991

题目分析：
简单的a+b问题，两个注意点：
(1)在考虑逗号的时候，是每遇到3个数字加一个逗号，但最后一个逗号不一定存在，如600000，如果不把最后的逗号去掉，就会成为，600，000
(2)考虑a+b=0的情况，如果是0，字符串为空，这时要处理一下。

代码如下：

 1 #include<iostream>
 2 #include<cstring>
 3 #include<algorithm>
 4 using namespace std;
 5 int main(){
 6     int a,b;
 7     cin>>a>>b;
 8     int c = a+b;
 9     int cc = c;
10     c = abs(c);
11     string s = "";
12     int count = 0;
13     while(c){
14         count++;
15         s += ('0'+c%10);
16         c /= 10;
17         if(count%3==0&&c){   //防止最后一位多出逗号，如600000，出现“,600,000”的情况 
18             s += ',';
19         }
20     }
21     if(cc<0){
22         s += '-';
23     }
24     reverse(s.begin(),s.end());
25     if(s == ""){            //注意判断和为0的情况，否则输出的s为空 
26         cout<<0<<endl;
27     } else{
28         cout<<s<<endl;
29     }
30     return 0;
31 }