用switch解自定义函数之字符提取

#include <stdio.h>

int main() {

	char arr[100];
	char brr[100];
	scanf("%s", arr, sizeof(arr));
	int len = strlen(arr);
	int j = 0;
	for (int i = 0; i < len; i++) {
		
		if (arr[i] == 'a' || arr[i] == 'e' || arr[i] == 'i' || arr[i] == 'o' || arr[i] == 'u') {
			brr[j] = arr[i];
			j++;
		}
	}
	int t = j;
	for (int i = 1; i < t; i++) {
		for (int k = 0; k <= t - 2; k++) {
			if (brr[k] > brr[k + 1]) {
				int temp = brr[k];
				brr[k] = brr[k + 1];
				brr[k + 1] = temp;
			}
		}
	}
	for (int i = 0; i < t; i++) {
		printf("%c", brr[i]);
	}

	return 0;
}

include<string.h>
void FlashBack(char arr1[],char arr2[])
{   
    int i,j,k;
    int num1=strlen(arr1);
    char arr3[100];
	for(i=0;i<num1;i++)
	{   
		for(j=0;j<5;j++)
		{
			if(arr1[i]==arr2[j])
			{
				
				   arr3[i]=arr1[i];
				   printf("%c",arr3[i]);
				
			}
		}
	}
}
int main()
{
	char arr1[100],arr2[5]={'a','e','i','o','u'};
    fgets(arr1, sizeof arr1, stdin);  
	FlashBack(arr1,arr2);
	return 0;
	
  }

char* haha(const char arr[],int a)
{
	char* b;
	int k = 0;
	int i, j;
	char arr2[5] = {'a','e','i','o','u'};
	static char arr3[100];
	for (i= 0; i < 5;i++)
	{
		for (j = 0;j < a ; j++)
		{
			if (arr[j] == arr2[i])
			{
				arr3[k] = arr[j];
				k++;
			}
		}

	}
	b = &arr3;
	return b;
}
int main()
{
	char* b;
	int a, i;
	char arr1[100];
	scanf("%s,",&arr1);
	a = strlen(arr1);
	b = haha(arr1,a);
	for (i = 0; i < a; i++)
	{
		printf("%c",b[i]);
	}
	return 0;
}
来个大佬看看，答案试了很多次都是对的，但是交上去说答案不对

#include <stdio.h>
#include <string.h>
int f(char a[],char b[])
{
    int i,j;
    char x;
    scanf("%s",a);
    int len=strlen(a);
    for(i=0;i<len;i++)
    {
        if(a[i]=='a'||a[i]=='e'||a[i]=='i'||a[i]=='o'||a[i]=='u')
        a[i]=b[i];
    }
    int len1=strlen(b);
    for(i=0;i<len1;i++)
    {
        for(j=i;j<len1;j++)
        {
            if(a[i]>a[j])
            {
                x=a[j];a[i]=a[j];x=a[i];
            }
        }
    }
    return 0;
}
int main()
{
    char m[100],n[100];
    scanf("%s",m);
    f(m,n);
    printf("%s",n);
    return 0;
}
大佬，我这个哪有问题，求指点。

为什么第一个函数的return后面没有0啊