C++大数加法

#include <iostream>
#include<deque>
#include<string>

using namespace std;

string add(string a, string b)   //此函数默认a的长度大于b(可以在main函数里用if语句控制a的长度大于b)
{
    deque<int>sum;        //sum用来存储和的每一位
    int carry = 0;        //carry表示进位
    int i = a.length()-1;
    int j = b.length()-1;
    for(; i >= 0,j >= 0; --i,--j)
    {
        int wei_sum = (a[i] - '0') + (b[j] - '0') + carry;   //wei_sum表示a,b每个对应相同的位相加的和
        if(wei_sum >= 10)
        {
            carry = 1;
            sum.push_front(wei_sum-10);
        }
        else
        {
            carry = 0;
            sum.push_front(wei_sum);
        }

    }
    for(int k = a.length()-b.length()-1; k >= 0; --k)        //算a比b长的那部分的和
    {
        int wei_sum = (a[k] - '0') + carry;            //此处一定要加上之前的进位值！因为之前的carry有可能为1.
        if(wei_sum >= 10)
        {
            carry = 1;
            sum.push_front(wei_sum-10);
        }
        else
        {
            carry = 0;
            sum.push_front(wei_sum);
        }
    }
    if(carry == 1)
        sum.push_front(1);

    string Sum;
    for(auto d: sum)
        Sum = Sum + char(d + '0');  //把sum转化为string
    return Sum;
}



int main()
{
    string a,b;
    cin >> a >> b;
    if(a.length() > b.length())
        cout << a << " + " << b << " = " << add(a,b);
    else
        cout << a << " + " << b << " = " << add(b,a);

    return 0;
}