PAT菜鸡笔记1001 A+B Format

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 −10​6​​≤a,b≤10​6​​. 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

注意点：

1. 先判断正负，然后都按正数办
2. 逗号打印是从后往前打印的，所以使用i–。

#include<iostream>
#include<vector>
using namespace std;
int main() {
 int a, b;
 vector<int> s;
 scanf("%d %d", &a, &b);
 int sum = a + b;
 if (sum == 0)
 {
  printf("0\n");
  return 0;
 }
 if (sum < 0)
 {
  sum = -sum;
  printf("-");
 }
 while (sum != 0)
 {
  a = sum % 10;
  sum /= 10;
  s.push_back(a);
 }
 for (int i = s.size() - 1; i >= 0; i--)
 {
  printf("%d", s[i]);
  if (i % 3 == 0 && i != 0)
   printf(",");
 }
 printf("\n");
 return 0;
}