用c语言或者c++编程

写一个函数，输入一个十六进制数，输出相应的十进制数。

#include <stdio.h>
#include <math.h>
//16进制转10进制
int main(void)
{
    char a[20];     
    int b[20] = {0}; 
    int i = 0, j = 0;
    int sum = 0;
    int c = 0; 

    gets(a);
    while (a[sum] != '\0')
    {
        if ((a[sum] >= 'a') && (a[sum] <= 'z'))
        {
            b[sum] = a[sum] - 'a' + 10;
            sum++;
            continue;
        }
        if ((a[sum] >= 'A') && (a[sum] <= 'Z'))
        {
            b[sum] = a[sum] - 'A' + 10;
            sum++;
            continue;
        }
        b[sum] = a[sum] - '0';
        sum++;
    }

    for (i = 0; i < sum; i++)
    {
        b[sum - i - 1] = b[sum - i - 1] * pow(16, i);
    }

    for (j = 0; j < sum; j++)
    {
        c = c + b[j];
    }
    printf("%d", c);

    return 0;
}