大意是输入一串字符串，按照ASC码从小到大排列！
错误代码如下：
#include"stdio.h"
//#include"string.h"
void main()
{
    int i,k,n;
    char a[20];
    char b;
   
    printf("please enter the total num:\n");
    scanf("%d",&n);

    printf("please enter you srting:\n");   
    //gets(a);
    scanf("%s",a);

    for(i=0; i<n-1; i++)
    {
        for(k=i+1;k<n;k++)
        {
            //c=a[0];

            if(a[i]>a[k])
            {
                b=a[i];
                a[i]=a[k];
                a[k]=b;
            }
        }
    }
    for(i=0;i<n;i++);
    {
        printf("%c",a[i]);
    }
}