为什么每一次跟题解算出来的一样，到自己的提交就是都错

#include <stdio.h>
#include <vector>
#include <algorithm>
#include <iostream>
using namespace std;

int main()
{
    vector<int> v1, v2;
    char c;
    while (scanf("%c", &c))
    {
        if (c == '\n')      break;
        v1.push_back(c - '0');
    }
    while (scanf("%c", &c))
    {
        if (c == '\n')      break;
        v2.push_back(c - '0');
    }
    int i = v1.size(), j = v2.size();
    if (i < j)
    {
        swap(v1, v2);
        swap(i, j);
    }

    vector<int> v3(i, 0);
    int k,l,a,b,count;
    for (j--,count=0; j >= 0;count++,j--)
    {
        for (k = i-1,a=0,b=0,l = v3.size()-1-count; l>=0; k--,l--)
        {
            a = v1[k] * v2[j] + a;
            b = v3[l] + a % 10+b;
            v3[l] = b % 10;
            a /= 10;
            b /= 10;
        }
        while(k>=0)
        {
            v3.insert(v3.begin(), 0);
            a = v1[k] * v2[j] + a;
            b = v3[0] + a % 10+b;
            v3[0] = b % 10;
            a /= 10;
            b /= 10;
            k--;
        }
        while (a != 0 || b != 0)
        {
            v3.insert(v3.begin(), 0);
            b = v3[0] + a % 10 + b;
            v3[0] = b % 10;
            a /= 10;
            b /= 10;
        }
    }
    if (v3[0] == 0)
    {
        cout << v3[0];
        return 0;
    }
    for (auto it : v3)
        cout << it;
    return 0;
}