怎么判断两个String的交集

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

void main()
{
    char  r0[256];
    char  r1[256];
    char buff[1024];
    char *b0;
    int  i;

    memset(r0, 0, 256);
    puts("pls input the first string:");
    gets(buff);
    for (b0 = buff; *b0; b0++)
    {
        if (*b0 && !r0[*b0])
            r0[*b0] = 1;
    }
    memset(r1, 0, 256);
    puts("pls input the second string:");
    gets(buff);
    for (b0 = buff; *b0; b0++)
    {
        if (*b0 && !r1[*b0])
            r1[*b0] = 1;
    }

    for (i = 0; i < 256; i++)
        if (r0[i] && r1[i])
            putc(i, stdout);
    putc(0x0d, stdout);
}