typedef enum {
    red, white, blue
} color;

void swap(color *a, color *b) {
    int t;
    t = *a;
    *a = *b;
    *b = t;
}

void flag(color a[], int n) {
    int i = 0, j = 0, k = n - 1;
    while (j <= k) {
        switch (a[j]) {
            case red:
                swap(&a[i++], &a[j++]);
                break;
            case white:
                j++;
                break;
            case blue:
                swap(&a[j], &a[k--]);
        }
    }
}