#include <stdio.h>
#include <math.h>

int convertBinarytoOctal(long long binaryNumber);
int main()
{
    long long binaryNumber;

    printf("输入一个二进制数: ");
    scanf("%lld", &binaryNumber);

    printf("二进制数 %lld 转换为八进制为 %d", binaryNumber, convertBinarytoOctal(binaryNumber));

    return 0;
}

int convertBinarytoOctal(long long binaryNumber)
{
    int octalNumber = 0, decimalNumber = 0, i = 0;

    while(binaryNumber != 0)
    {
        decimalNumber += (binaryNumber%10) * pow(2,i);
        ++i;
        binaryNumber/=10;
    }

    i = 1;

    while (decimalNumber != 0)
    {
        octalNumber += (decimalNumber % 8) * i;
        decimalNumber /= 8;
        i *= 10;
    }

    return octalNumber;
}

输入一个二进制数: 101001
二进制数 101001 转换为八进制为 51

#include <stdio.h>
#include <math.h>

long long convertOctalToBinary(int octalNumber);
int main()
{
    int octalNumber;

    printf("输入一个八进制数: ");
    scanf("%d", &octalNumber);

    printf("八进制数 %d 转二进制为 %lld", octalNumber, convertOctalToBinary(octalNumber));

    return 0;
}

long long convertOctalToBinary(int octalNumber)
{
    int decimalNumber = 0, i = 0;
    long long binaryNumber = 0;

    while(octalNumber != 0)
    {
        decimalNumber += (octalNumber%10) * pow(8,i);
        ++i;
        octalNumber/=10;
    }

    i = 1;

    while (decimalNumber != 0)
    {
        binaryNumber += (decimalNumber % 2) * i;
        decimalNumber /= 2;
        i *= 10;
    }

    return binaryNumber;
}

输入一个八进制数: 51
八进制数 51 转二进制为 101001