#include<bits/stdc++.h>
using namespace std;
int main()
{
    string s;
    cin >> s;
    long long sum = 0;
    for (int i = 0;i < s.length();i++)
    {
        if (s[i] != '0')
        {
            if (s[i] >= '1' && s[i] <= '9')
            {
                sum += (s[i] - '0') * pow(16, s.length() - i - 1);
            }
            else if (s[i] == 'A')
            {
                sum += 10 * pow(16, s.length() - i - 1);
            }
            else if (s[i] == 'B')
            {
                sum += 11 * pow(16, s.length() - i - 1);
            }
            else if (s[i] == 'C')
            {
                sum += 12 * pow(16, s.length() - i - 1);
            }
            else if (s[i] == 'D')
            {
                sum += 13 * pow(16, s.length() - i - 1);
            }
            else if (s[i] == 'E')
            {
                sum += 14 * pow(16, s.length() - i - 1);
            }
            else if (s[i] == 'F')
            {
                sum += 15 * pow(16, s.length() - i - 1);
            }
        }
    }
    cout << sum;
    return 0;
}

#include<iostream>
#include<cmath>
using namespace std;
string n;
long long a=0;
int main(){
	cin>>n;
	for(int i=n.length()-1,j=0;i>=0;i--,j++){
		int t;
		switch(n[i]){
			case 'A':t=10;break;
			case 'B':t=11;break;
			case 'C':t=12;break;
			case 'D':t=13;break;
			case 'E':t=14;break;
			case 'F':t=15;break;
			default:t=n[i]-'0';
		}
		a+=t*pow(16,j);
	}
	cout<<a<<endl;
	return 0;
}

思路和MOD的一样，但是可能更简洁吧
已AC

#include <bits/stdc++.h>
using namespace std;
int getnum(char c){
    if(c>='0'&&c<='9')return c-'0';
    else{
        switch(c){
            case 'A':return 10;
            case 'B':return 11;
            case 'C':return 12;
            case 'D':return 13;
            case 'E':return 14;
            case 'F':return 15;
        }
    }
}
int main(){
    long long ans=0;
    string s;
    cin>>s;
    for(int i=0;i<s.length();i++)ans+=pow(16,s.length()-i-1)*getnum(s[i]);
    cout<<ans<<endl;
    return 0;
}

#include<bits/stdc++.h>
using namespace std;
int main()
{
    string s;
    cin >> s;
    long long sum = 0;
    for (int i = 0;i < s.length();i++)
    {
        if (s[i] != '0')
        {
            if (s[i] >= '1' && s[i] <= '9')
            {
                sum += (s[i] - '0') * pow(16, s.length() - i - 1);
            }
            else if (s[i] == 'A')
            {
                sum += 10 * pow(16, s.length() - i - 1);
            }
            else if (s[i] == 'B')
            {
                sum += 11 * pow(16, s.length() - i - 1);
            }
            else if (s[i] == 'C')
            {
                sum += 12 * pow(16, s.length() - i - 1);
            }
            else if (s[i] == 'D')
            {
                sum += 13 * pow(16, s.length() - i - 1);
            }
            else if (s[i] == 'E')
            {
                sum += 14 * pow(16, s.length() - i - 1);
            }
            else if (s[i] == 'F')
            {
                sum += 15 * pow(16, s.length() - i - 1);
            }
        }
    }
    cout << sum;
    return 0;
}

#include<iostream>
#include<cmath>
#include<cstring>
using namespace std;
int main(void)
{
    int n;
    int temp;
    char* arr = new char[9];
    cin >> arr;
    n = strlen(arr);
    long long sum = 0;
    for (int i = 0; arr[i]!='\0'; i++)
    {
        switch (arr[i])
        {
        case 'A': temp = 10; break;
        case 'B': temp = 11; break;
        case 'C': temp = 12; break;
        case 'D': temp = 13; break;
        case 'E': temp = 14; break;
        case 'F': temp = 15; break;
        default: temp = arr[i] - '0'; break;
        }
        sum = sum + temp * pow(16, n - 1- i);
    }
    cout << sum;
    return 0;
}