求为什么没有输出（XJOI 7716）

Question

题目：
有n个人，每个人都拥有一个喜悦值，接下来，任意两个人之间会进行一次交流，任意两个人交流所获得的贡献为两个人的喜悦值的和，求任意两个人交流所获得的贡献之和
输入格式：
第一行一个整数n
接下来n行，每行一个整数a，表示第i个人的喜悦值
输出格式：
按题目描述输出
样例输入1：
3
2
3
4
样例输出1：
18
约定：
n<=1000
1<=a<=10^100
这是我的代码是方法那里搞错了吗

#include<bits/stdc++.h>
using namespace std;
string str;
void q(string a,string b){
    int a1[1001]={},b1[1001]={},c1[1001]={};
    int lena=a.size(),lenb=b.size();
    int len=max(lena,lenb);
    for(int i=0;i<lena;i++){
        a1[i]=a[lena-i-1]-'0';
    }
    for(int i=0;i<lenb;i++){
        b1[i]=b[lenb-i-1]-'0';
    }
    int x=0;
    for(int i=0;i<=len;i++){
        c1[i]=a1[i]+b1[i]+x;
        if(c1[i]>=10){
            c1[i]-=10;
            x=1;
        }else{
            x=0;
        }
    }
    if(c1[len]==0) len--;
    for(int i=len;i>=0;i--){
        str[len-i]=c1[i]+48;
    }
}

int main(){
    int n;
    string s;
    cin>>n;
    for(int i=0;i<n;i++){
        cin>>s;
        q(str,s);
    }
    cout<<str;
    return 0;
}

反正莫输出，help me！

Answer 1

#include <iostream>
#include <string>
#include <vector>

class BigInt
{
public:
    BigInt(unsigned int n = 0)
    {
        if (n == 0)
            _data.push_back('0');
        else
        {
            std::string str;
            while (n)
            {
                str.push_back(n % 10 + '0');
                n /= 10;
            }
            _data.assign(str.rbegin(), str.rend());
        }
    }

    BigInt(const std::string &s) : _data(s) {}

private:
    std::string _data;

    friend BigInt operator+(const BigInt &lsh, const BigInt &rhs);
    friend std::istream &operator>>(std::istream &is, BigInt &a);
    friend std::ostream &operator<<(std::ostream &os, const BigInt &a);
};

BigInt operator+(const BigInt &lhs, const BigInt &rhs)
{
    std::string r;
    int carry = 0;
    auto itr1 = lhs._data.rbegin();
    auto itr2 = rhs._data.rbegin();
    while (itr1 != lhs._data.rend() && itr2 != rhs._data.rend())
    {
        int a = *itr1 - '0';
        int b = *itr2 - '0';
        int c = a + b + carry;
        if (c >= 10)
        {
            carry = c / 10;
            c %= 10;
        }
        else
        {
            carry = 0;
        }
        r.push_back(c + '0');
        ++itr1;
        ++itr2;
    }
    while (itr1 != lhs._data.rend())
    {
        int a = *itr1 - '0' + carry;
        if (a >= 10)
        {
            carry = a / 10;
            a %= 10;
        }
        else
        {
            carry = 0;
        }
        r.push_back(a + '0');
        ++itr1;
    }
    while (itr2 != rhs._data.rend())
    {
        int a = *itr2 - '0' + carry;
        if (a >= 10)
        {
            carry = a / 10;
            a %= 10;
        }
        else
        {
            carry = 0;
        }
        r.push_back(a + '0');
        ++itr2;
    }
    if (carry > 0)
        r.push_back(carry + '0');
    return BigInt(std::string(r.rbegin(), r.rend()));
}

std::istream &operator>>(std::istream &is, BigInt &a)
{
    is >> a._data;
    return is;
}

std::ostream &operator<<(std::ostream &os, const BigInt &a)
{
    os << a._data;
    return os;
}

int main()
{
    int n;
    std::cin >> n;
    std::vector<BigInt> a(n);
    for (int i = 0; i < n; i++)
        std::cin >> a[i];
    BigInt sum = 0;
    for (int i = 0; i < n - 1; i++)
        for (int j = i + 1; j < n; j++)
            sum = sum + a[i] + a[j];
    std::cout << sum;
    return 0;
}