C :

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

char s[10000001];//s为输入的十六进制 
char t[1];
char n[10000001]; 
int main()
{
	scanf("%s",&s);
	int len = strlen(s);//获取十六进制的长度 
	long long i,j;
	long long x,sum=0;
	for(i=0;i<len;i++){
		if(isdigit(s[i])!=0){     //如果为数字 
			x=s[i]-'0';             
		}else {
			x=s[i]-'A'+10;    //如果为字母 
		}
		for(j=1;j<=len-i-1;j++){
			x=x*16;
		}
		sum+=x;                      //此时sum为整型 
	}
	if(sum==0){
		printf("%d",0);
		return 0;
	}
	while(sum!=0){
		t[0]=sum%8+'0';           //t[0]为字符型 
		strcat(n,t); 			 //拼接字符串 
		sum/=8;           
	}
	for(i=strlen(n)-1;i>=0;i--){
		printf("%c",n[i]);
	} 
	return 0;
 } 

C++ :

#include <bits/stdc++.h>
using namespace std;
string num (int n)
{
    string s[]={"0000","0001","0010","0011","0100","0101","0110","0111","1000","1001","1010","1011","1100","1101","1110","1111"};
    return s[n];
}
char fun(string a)
{
    int k=1,s=0;
    for (int i=2;i>=0;i--)
    {
        s=s+(a[i]-48)*k;
        k=k*2;
    }
    return char(s+48);
}
int main()
{
    string r,a,b;//b存放二进制 
    cin>>a;//十六进制数 
     
    for (int i=0;i<a.size();i++)
    {
        if (a[i]>='A') b=b+num(a[i]-55);
        else b=b+num(a[i]-48);
    }
    
    //删除前导0
	while(b[0] == '0' && b.size() != 1) {
		b.erase(0,1);
	}
    int t=b.size();
    if (t%3!=0)
    {
        for (int i=1;i<=3-t%3;i++)
        {
            b="0"+b;
        }
    }
    
    //2进制转8进制 
    for (int i=0;i<b.size();i+=3)
    {
        r=r+fun(b.substr(i,3));
    }
    if (a!="0") cout<<r<<endl;
    else cout<<0<<endl;
    return 0;
}

Python :

n=input()
a=int(n,16)
b=oct(a)[2:]
print(b)