我之前提交的代码

vector<int> myadd(vector<int>& va, vector<int>&vb) {
    if (va.size() == 0) return vb;
    if (vb.size() == 0) return va;
    int carry = 0;
    int i = 0, j = 0;
    vector<int> vc;
    while(i < va.size() && j < vb.size()) {
        int res = va[i] + vb[j] +carry;
        if (res > 9) {
            res -= 10;
            carry = 1;
        } else {
            carry = 0;
        }
        vc.push_back(res);
        i ++;
        j ++;
    }
    while(i < va.size()) {
        int res = va[i] + carry;
        if (res > 9) {
            res -= 10;
            carry = 1;
        } else {
            carry = 0;
        }
        vc.push_back(res);
        i ++;
    }
    while(j < vb.size()) {
        int res = vb[j] + carry;
        if (res > 9) {
            res -= 10;
            carry = 1;
        } else {
            carry = 0;
        }
        vc.push_back(res);
        j ++;
    }
    if (carry) {
        vc.push_back(carry);
    }
    return vc;
}

简单好懂，易于调试，适合初学者。

模板代码

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

简洁高效，适合背诵。