…乘法在进行尝试中，大概的思路有了
加法的函数
string addStrings(string num1, string num2) {
if (num1.empty()) return num2;
if (num2.empty()) return num1;
if (num1.front() == '0') return num2;
if (num2.front() == '0') return num1;
int sz1 = num1.size(), sz2 = num2.size();
int r, tmp = 0;
string out;
if (sz1 >= sz2) {
for (r = 0; r < sz2; ++r) {
out.push_back(num1[sz1 - r - 1] - '0' + num2[sz2 - r - 1] + tmp);
if (out.back() > '9') {
tmp = 1;
out[out.size() - 1] = out[out.size() - 1] - 10;
}
else tmp = 0;
}
while (tmp == 1 && r < sz1) {
out.push_back(tmp + num1[sz1 - r - 1]);
if (out[out.size() - 1] > '9') {
tmp = 1;
out.back() = out.back() - 10;
}
else tmp = 0;
++r;
}
if (tmp == 1 && r >= sz1) {
out.push_back('1');
}
for (; r < sz1; ++r) {
out.push_back(num1[sz1 - r - 1]);
}
}
else {
for (r = 0; r < sz1; ++r) {
out.push_back(num1[sz1 - r - 1] - '0' + num2[sz2 - r - 1] + tmp);
if (out[out.size() - 1] > '9') {
tmp = 1;
out[out.size() - 1] = out[out.size() - 1] - 10;
}
else tmp = 0;
}
while (tmp == 1 && r < sz2) {
out.push_back(tmp + num2[sz2 - r - 1]);
if (out[out.size() - 1] > '9') {
tmp = 1;
out[out.size() - 1] = out[out.size() - 1] - 10;
}
else tmp = 0;
++r;
}
if (tmp == 1 && r >= sz2) {
out.push_back('1');
}
for (; r < sz2; ++r) {
out.push_back(num2[sz2 - r - 1]);
}
}
char ctmp;
sz1 = out.size();
for (r = 0; r < sz1 / 2; ++r) {
ctmp = out[sz1 - r - 1];
out[sz1 - r - 1] = out[r];
out[r] = ctmp;
}
return out;
}