K&RC程序设计语言书中Hashtable的有效性

在K&R的C程序设计语言中，Hashtable的实现非常简单且易于理解。不过，它的有效性在某些情况下会受到影响。

下面是Hashtable的代码示例：

#define HASHSIZE 101

typedef struct ListNode ListNode;
typedef struct Hashtable Hashtable;

struct ListNode {
    char *key;
    char *value;
    ListNode *next;
};

struct Hashtable {
    ListNode *table[HASHSIZE];
};

unsigned hash(char *s) {
    unsigned hashval;
    for (hashval = 0; *s != '\0'; s++)
        hashval = *s + 31 * hashval;
    return hashval % HASHSIZE;
}

ListNode *lookup(Hashtable *htable, char *key) {
    ListNode *np;
    for (np = htable->table[hash(key)]; np != NULL; np = np->next)
        if (strcmp(key, np->key) == 0)
            return np; // if found
    return NULL; // if not found
}

ListNode *addnode(Hashtable *htable, char *key, char *value) {
    ListNode *np;
    unsigned hashval;
    if ((np = lookup(htable, key)) == NULL) { // if not already in the hash table
        np = (ListNode *)malloc(sizeof(*np));
        if (np == NULL || (np->key = strdup(key)) == NULL)
            return NULL;
        hashval = hash(key);
        np->next = htable->table[hashval];
        htable->table[hashval] = np;
    } else {
        free((void *)np->value); // free the old value
    }
    if ((np->value = strdup(value)) == NULL)
        return NULL;
    return np;
}

void deletenode(Hashtable *htable, char *key) {
    ListNode *np, *prev;
    unsigned hashval = hash(key);
    for (np = htable->table[hashval], prev = NULL; np != NULL; prev = np, np = np->next) {
        if (strcmp(key, np->key) == 0) {
            if (prev == NULL) {
                h