#include <stdio.h>
void dec_to_bin(char s[],int num){
    int a[100],m,i=0,j,k;
    while(1){
        m=num%2;
        a[i]=m;
        num=num/2;
        if(num==0)break;
        i++;
    }
    k=i;j=0;
        for(;j<=k;j++,i--){
            s[j]=a[i];
        }
}
int bin_to_dec(const char s[]){
  int i = 0, n = 0;
  while(s[i]!='\0'&&(s[i]=='0'||s[i]=='1')){
    n = n * 2 + (s[i] - '0');
    ++i;
  }
  return n;
}
main(){
    char string[100]={'\0'};
    int in_num,out_num;
   printf("输入一个十进制数:");
   scanf("%d",&in_num);
   dec_to_bin(string,in_num);
   printf("二进制是：%s\n",string);
   out_num=bin_to_dec(string);
   printf("十进制是：%d\n",out_num);
}

修改了一下你的十进制转二进制程序
那个2转10的我没动

#include <stdio.h>
#include <string.h>
void dec_to_bin(char s[],int num)
{
    char s2[100];
    int m,i=0;
    while(1)
    {
        m=num%2;
        s2[i]=m+48;
        num=num/2;
        if(num==0)break;
        i++;
    }
    int j=0;
    while(1)
    {
        if(i<0) break;
        s[j]=s2[i];
        j++;
        i--;
    }

 }
int bin_to_dec(const char s[])
{
  int i = 0, n = 0;
  while(s[i]!='\0'&&(s[i]=='0'||s[i]=='1'))
  {
    n = n * 2 + (s[i] - '0');
    ++i;
  }
  return n;
}
int main(int argc,char *argv[])
{
    char string[100];
    for(int i=0;i<100;i++) string[i]='\0';
    int in_num,out_num;
    printf("输入一个十进制数:");
    scanf("%d",&in_num);
    dec_to_bin(string,in_num);
    printf("二进制是：%s\n",string);
    out_num=bin_to_dec(string);
    printf("十进制是：%d\n",out_num);
    return 0;
}

[ 本帖最后由 wp231957 于 2012-11-19 17:55 编辑 ]