十六进制转换为十进制【C++】

#include <iostream>
#include <cstring>
#include <algorithm>
#include <cmath>

using namespace std;
string str;
int n, m;

int main()
{
    getline(cin, str);
    cout << str << endl;
    int n = str.size();
    int sum=0;
    for (int i=0;i < n; i ++)  //遍历该数的每一位！
    {
        int nums = 0;
        if (str[i]-'0' >= 10){  //ABCDEF
            if (str[i]=='A') nums = 10;
            else if (str[i] == 'B') nums = 11;
            else if (str[i] == 'C') nums = 12;
            else if (str[i] == 'D') nums = 13;
            else if (str[i] == 'E') nums = 14;
            else nums = 15;
        }
        else{
            nums = str[i] - '0';    //数字字符转化为int类型的数字！
        }
        sum = sum + nums * pow(16, n-i-1);
    }
    cout << sum << endl;
    return 0;
}