[编程题]大数的运算

题目：输入某个数，计算它乘以2的结果

#include <iostream>
#include <string>
#pragma warning(disable : 4996)
using namespace std;

string multiply2(string &str) {
    int len = str.size();
    if (len == 0) return "0";
string result = str;

    int location = len - 1; //从最后一位开始计算
    int locationSum = 0;    //当前位置的数相加的结果
    int carry = 0;          //进位
    int curr = 0;           //计算后该位的实际值
    char buffer[2];

    while (location >= 0) {
        if (str[location] >= '0' && str[location] <= '9') {
            
            locationSum = (str[location] - '0') * 2  + carry;

            if (locationSum > 9) {
                curr = locationSum - 10;
                itoa(curr, buffer, 10);
                result[location] = buffer[0];
                carry = 1;
            }
            else {
                curr = locationSum;
                itoa(curr, buffer, 10);
                result[location] = buffer[0];
                carry = 0;
            }
        }
        else { //输入不合法
            cerr << "invalid input" << endl;
            return "0";
        }

        location--;
    }

    if (carry == 1) {
        string big = "1"; //最终的进位
        result = big + result;
    }

    return result;

}

int main() {
    string a;
    getline(cin, a);
    string b = multiply2(a);
    cout << b << endl; 
    return 0; 
}