题解 | #Powerful Calculator#

#include <iostream>
#include <vector>
using namespace std;

vector<int> a,b,add_c, sub_c, mul_c;

vector<int> add(vector<int> &a, vector<int> &b)
{
    if(a.size() < b.size()) return add(b,a);
    int t = 0;
    for(int i = 0; i < a.size(); i++)
    {
        t += a[i];
        if(i < b.size()) t += b[i];
        add_c.push_back(t % 10);
        t /= 10;
    }
    if(t) add_c.push_back(t);
    return add_c;
}

bool cmp(vector<int> &a, vector<int> &b)
{
    if(a.size() != b.size()) return a.size() > b.size();
    for(int i = a.size() - 1; i >= 0; i--)
        if(a[i] != b[i])
            return a[i] > b[i];
    return true;
}

vector<int> sub(vector<int> &a, vector<int> &b)
{
    int t = 0;
    for(int i = 0; i < a.size(); i++)
    {
        t = a[i] - t;
        if(i < b.size()) t -= b[i];
        sub_c.push_back((t+10) % 10);
        if(t < 0) t = 1;
        else t = 0;
        
    }
    
    while(sub_c.size() > 1 && sub_c.back() == 0) sub_c.pop_back();
    return sub_c;
}

vector<int> mul(vector<int> &a, vector<int> &b)
{
    vector<int> mul_c(max(a.size(), b.size()) + 100, 0);
    for(int i = 0; i < a.size(); i++)
    {
        for(int j = 0; j < b.size(); j++)
        {
            mul_c[i+j] += a[i] * b[j]; 
        }
    }

    for(int i = 0; i < mul_c.size(); i++)
    {
        mul_c[i+1] += mul_c[i] / 10;
        mul_c[i] %= 10;
    }
    while(mul_c.size() > 1 && mul_c.back() == 0) mul_c.pop_back();
    return mul_c;
}

int main() {
    string m, n;
    cin >> m >> n;

    for(int i = m.size() - 1; i >= 0; i--) a.push_back(m[i] - '0');
    for(int i = n.size() - 1; i >= 0; i--) b.push_back(n[i] - '0');

    //加法
    add_c = add(a,b);
    for(int i = add_c.size() - 1; i >= 0 ; i--)  cout << add_c[i];
    cout << endl;

    //减法
    if(cmp(a, b)) sub_c = sub(a, b);
    else 
    {
        sub_c = sub(b, a) ;
        cout << '-';
    }
    for(int i = sub_c.size() - 1; i >= 0 ; i--)  cout << sub_c[i];
    cout << endl;

    //乘法
    mul_c = mul(a, b);
    for(int i = mul_c.size() - 1; i >= 0 ; i--)  cout << mul_c[i];
    cout << endl;



    
}
// 64 位输出请用 printf("%lld")