字符串的排序及交换

问题
编写一个函数将一条字符串分成两部分，将前半部分按ASCII码升序排列，后半部分不变，再将前后两部分交换，最后将该字符串输出。测试字符串“ADZDDJKJFIEJHGI”.
代码

#include <stdio.h>#include <stdlib.h>//冒泡排序算法void mysort(char *str,int num){    int i,j;    int temp = 0;    for (i = 0;i<num;i++)    {        for(j = i +1;j<num;j++)        {            if(str[i]<str[j])            {                temp = str[i];                str[i] = str[j];                str[j] = temp;            }        }    }}char *foo(char *str){    int len = 0;    char *start = NULL:    if(str == NULL)    {        return NULL:    }    start = str;    while(*str++);    len = str - start -1;    len = len/2;    str = start;    mysort(str,len);    return str;}int main(){    char string[] = "ADZDDJKJFIEJHGI";    printf("%s\n",string);    foo(string);    printf("%s\n",string);    return 0;}