并查集

typedef struct ufs_elem_st {
    struct ufs_elem_st *next, *prev;
    struct ufs_elem_st *parent;
} ufs_elem_st, *ufs_elem_t;

typedef struct ufs_st {
    ufs_elem_st *roots;
} ufs_st, *ufs_t;

ufs_elem_t ufs_get_root(ufs_elem_t me) {
    ufs_elem_t my_root = me;
    while( my_root->parent != my_root && my_root->parent != NULL ) 
        my_root = my_root->parent;

    me->parent = my_root; //路径压缩
    return my_root;
}

void ufs_add_root(ufs_t ufs, ufs_elem_t me) {
    ufs_elem_t my_root = ufs_get_root(me);
    if( NULL != ufs->roots ) {
        my_root->next = ufs->roots;
        ufs->roots->prev = my_root;
    }
    ufs->roots = my_root;
    my_root->prev = NULL;
}

void ufs_join(ufs_t ufs, ufs_elem_t friend_elem, ufs_elem_t me) {
    ufs_elem_t my_root = ufs_get_root(me);
    ufs_elem_t friend_root = ufs_get_root(friend_elem);
    
    if( NULL != my_root->next )
        my_root->next->prev = my_root->prev;
    if( NULL != my_root->prev )
        my_root->prev->next = my_root->next;
    if( ufs->roots == my_root )
        ufs->roots = my_root->next;
    my_root->next = my_root->prev = NULL;

    if( NULL != friend_root->next )
        friend_root->next->prev = friend_root->prev;
    if( NULL != friend_root->prev )
        friend_root->prev->next = friend_root->next;
    if( ufs->roots == friend_root ) 
        ufs->roots = friend_root->next;
    friend_root->next = friend_root->prev = NULL;

    //合并集合
    friend_root->parent = my_root; 

    if( NULL != ufs->roots ) {
        my_root->next = ufs->roots;
        ufs->roots->prev = my_root;
    }
    ufs->roots = my_root;
    my_root->prev = NULL;
}