萬能進制轉換

a 爲輸入數的進制
b 爲要轉化的進制

16 進制變 2， 8，10
反之不行

#include<cstdio>
#include<cstring>
#include<cmath>
#include<iostream>

using namespace std;

typedef unsigned long long ll;
const ll MAXN = 10000;
char str[10000];
int ans[10000];
ll a, b;

int change(char c) {
    if(c >= '0' && c <= '9') return c - '0';
    else if(c >= 'a' && c <= 'f') return (c - 'a' + 10);
    else if(c >= 'A' && c <= 'F') return (c - 'A' + 10);
    return 0;
}

int main() {
    a = 16;
    b = 8;
    ll kase, flag, result;
    kase = 0, flag = 0, result = 0;
    scanf("%s", str);
    int len = strlen(str);
    for(int i = 0; i < len; i++)
        result += pow(a, len-i-1) * change(str[i]);
    if(b == 10) printf("%lld\n", result);
    else {
        while(result) {
            ans[kase++] = result % b;
            result /= b;
        }
        for(int i = kase-1; i >=0; i--)
            printf("%d", ans[i]);
        printf("\n");
    }
    return 0;
}