bitmap

bitmap.c

#define N 1000000   //the max value
#define MASK 0x1F

int a[1+N/32];  //int has 32 bits
void set(int i) {
    a[i>>5] |= (1 << (i & MASK));
}

void clear(int i) {
    a[i>>5] &= ~(1 << (i & MASK));
}

int is_existed(int i) {
    return a[i>>5] & (1 << (i & MASK));
}

I take it as a special hashtable