题解 | #Powerful Calculator#

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

vector<int> add(vector<int>& A, vector<int>& B) {
    int t = 0;
    vector<int> C;
    for (int i = 0; i < A.size() || i < B.size(); i++) {
        if (i < A.size()) t += A[i];
        if (i < B.size()) t += B[i];
        C.push_back(t % 10);
        t /= 10;
    }
    if (t) C.push_back(t);
    return C;
}
//判断是否A>=B
bool cmp(string& a, string& b) {
    if (a.size() != b.size()) return a.size() > b.size();
    else return a > b;
}
vector<int> sub(vector<int>& A, vector<int>& B) {
    int t = 0;
    vector<int> C;
    for (int i = 0; i < A.size(); i++) {
        t = A[i] - t;
        if (i < B.size()) t -= B[i];
        C.push_back((t + 10) % 10);
        if (t < 0) t = 1; //向高位的借位
        else t = 0;
    }
    //去除高位前导零
    while (C.size() > 1 && C.back() == 0) C.pop_back();
    return C;
}
vector<int> mul(vector<int>& A, vector<int>& B) {
    vector<int> C(A.size() + B.size(), 0);
    for (int i = 0; i < A.size(); i++) {
        for (int j = 0; j < B.size(); j++) {
            C[i + j] += A[i] * B[j];
        }
    }
    //处理进位
    for (int i = 0; i + 1 < C.size(); i++) {
        C[i + 1] += C[i] / 10;
        C[i] = C[i] % 10;
    }
    while (C.size() > 1 && C.back() == 0) C.pop_back();
    return C;
}
void print(vector<int>& res) {
    for (int i = res.size() - 1; i >= 0; i--) {
        cout << res[i];
    }
    cout << endl;
}

int main() {
    string a, b;
    cin >> a >> b;
    vector<int> A, B;
    for (int i = a.size() - 1; i >= 0; i--) A.push_back(a[i] - '0');
    for (int i = b.size() - 1; i >= 0; i--) B.push_back(b[i] - '0');
    //加法
    vector res_add = add(A, B);
    print(res_add);
    //减法
    if (cmp(a, b)) {
        vector res_sub = sub(A, B);
        print(res_sub);
    } else {
        cout << "-";
        vector res_sub = sub(B, A);
        print(res_sub);
    }
    //乘法
    vector<int> res_mul = mul(A, B);
    print(res_mul);
    return 0;
}