#include<iostream>
#include <stdio.h>
#include <stdlib.h>
using namespace std;
int main(){
int a,n[100];
while(scanf("%d",&a)!=EOF){
int i=0;
while((a!=1)&&(a!=0)){
n[i]=a%2;
a=(a-a%2)/2;
i++;
}
if(a==1){
n[i]=a;
}
i++;
while(i){
cout<<n[i-1];
i--;
}
cout<<endl;
}
}

cout 是 C++ 的，貼了一個純 C 的

#include <stdio.h>

const unsigned MAXSIZE = 100;

int main() {
  int num, size, ary[MAXSIZE];
  while (scanf("%d", &num) != EOF) {
    size = 0;
    do {
      ary[size++] = num % 2;
      num /= 2;
    } while (num > 0);

    for (size--; size >= 0; size--) printf("%d", ary[size]);

    puts("");
  }
  return 0;
}

#include 

const unsigned MAXSIZE = 100;

int main() {
  int num, size, ary[MAXSIZE];
  while (scanf("%d", &num) != EOF) {
    size = 0;
    do {
      ary[size++] = num % 2;
      num /= 2;
    } while (num > 0);

    for (size--; size >= 0; size--) printf("%d", ary[size]);

    puts("");
  }
  return 0;
}

為甚麼size--是for的初始值