Union, intersection and difference of 2 sets,using just string functions

Question

I have the following program that finds out the reunion, the intersection and the difference between the sets consisting of 2 strings. In my functions, I treat strings as one-dimensional arrays. I need a program that does the same thing, but that uses the maximum possible string processing functions. I'd be very glad if you could help me, I'm still a beginner. Here's my code below :

#include <stdio.h>
#include <string.h>


void stringToSet(char str[])
{
    int i, j, k;
    for (i = 0; i  < strlen(str); i++)
    {
        for (j = i + 1; j < strlen(str); j++)
        {
            if (str[j] == str[i])
            {
                for (k = j; str[k]; k++)
                {
                    str[k] = str[k + 1];
                }
            }
        }
    }
    printf("\nThe set is : %s\n", str);
}
void findUnion( char *str1, char *str2)
{
    int counter = 0;
    char str3[100];

    while (str1[counter])
    {
        str3[counter] = str1[counter];
        counter++;
    }
    for (int i = 0; i < strlen(str2); i++)
    {
        int flag = 0;
        for (int j = 0; j < strlen(str1); j++)
        {
            if (str2[i] == str1[j])
            {
                flag = 1;
            }

        }

        if (flag == 0)
        {
            str3[counter] = str2[i];
            counter++;
        }
    }
    str3[counter] = '\0';
    printf("\nUnion : ");
    (*str3) ? printf("%s", str3): printf("\n Empty set!");
}

void findIntersection(const char *str1, const char *str2)
{
    int k = 0;
    char str3[100];

    for (int i = 0; i < strlen(str1); i++)
    {
        for (int j = 0; j < strlen(str2); j++)
        {
            if (str1[i] == str2[j])
            {
                str3[k] = str1[i];
                k++;
            }
        }
    }
    str3[k] = '\0';
    printf("\nIntersection : ");
    (*str3) ? printf("%s", str3): printf("\n Empty set!");
}
void findDifferention(const char *str1, const char *str2)
{
    char str3[100];

    int  l = 0;

    for (int i = 0; i < strlen(str1); i++)
    {
        int flag = 0;
        for (int j = 0; j < strlen(str2); j++)
        {
            if (str1[i] == str2[j])
                flag = 1;
        }
        if (flag == 0)
        {
            str3[l] = str1[i];
            l++;
        }
    }

    str3[l] = '\0';
    printf("\nDiference :");
    (*str3) ? printf("%s", str3): printf("\n Empty set!");

}
int main()
{

    char str1[100];
    char str2[100];

    printf("String 1 :");
    fgets(str1,sizeof  str1, stdin);
    stringToSet(str1);

    printf("String  2 :");
    fgets(str2,sizeof  str2, stdin);
    stringToSet(str2);

    findUnion(str1, str2);
    findIntersection(str1, str2);
    findDifferention(str1, str2);

    return 0;
}