C :

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

char s[10001];//s为输入的八进制 
char t[1];
char n[10001]; 
int main()
{
	scanf("%s",&s);
	int len = strlen(s);//获取八进制的长度 
	long long i,j;
	long long x,sum=0;
	for(i=0;i<len;i++){
		x=s[i]-'0';
		for(j=1;j<=len-i-1;j++){
			x=x*8;
		}
		sum+=x;                      //此时sum为整型 
	}
	if(sum==0){
		printf("%d",0);
		return 0;
	}
	while(sum!=0){
		t[0]=sum%2+'0';           //t[0]为字符型 
		strcat(n,t); 			 //拼接字符串 
		sum/=2;           
	}
	for(i=strlen(n)-1;i>=0;i--){
		printf("%c",n[i]);
	} 
	return 0;
 } 

C++ :

#include <bits/stdc++.h>
using namespace std;

int main(){
	char s[110],r[500];
	char a[8][4] = {"000","001","010","011","100","101","110","111"};
	gets(s);
	int i,k = 0,x,j;
	for(i = 0;i < strlen(s);i++){
		x = s[i] - 48;
		for(j = 0;j < strlen(a[x]);j++){
			r[k] = a[x][j];
			k++;
		}
	}
	r[k] = '\0';
	
	int p = strlen(r) - 1;
	for(i = 0;i < strlen(r);i++){
		if(r[i] != '0'){
			p = i;
			break;
		}
	}
	
	for(i = p;i < strlen(r);i++){
		cout<<r[i];
	}
}

Python :

n=input()
a=int(n,8)
b=bin(a)[2:]
print(b)