题目：

题解：

#define HASH_FIND_CHAR(head, findint, out) HASH_FIND(hh, head, findint, sizeof(char), out)
#define HASH_ADD_CHAR(head, intfield, add) HASH_ADD(hh, head, intfield, sizeof(char), add)

struct HashTable {
    char key;
    int val;
    UT_hash_handle hh;
};

char* frequencySort(char* s) {
    struct HashTable* hashTable = NULL;
    int maxFreq = 0;
    int length = strlen(s);
    for (int i = 0; i < length; i++) {
        struct HashTable* tmp;
        HASH_FIND_CHAR(hashTable, &s[i], tmp);
        if (tmp == NULL) {
            tmp = malloc(sizeof(struct HashTable));
            tmp->key = s[i], tmp->val = 1;
            HASH_ADD_CHAR(hashTable, key, tmp);
            maxFreq = fmax(maxFreq, 1);
        } else {
            maxFreq = fmax(maxFreq, ++tmp->val);
        }
    }
    char* buckets[maxFreq + 1];
    int bucketsSize[maxFreq + 1];
    memset(bucketsSize, 0, sizeof(bucketsSize));
    int retSize = 0;
    struct HashTable *tmp, *iter;
    HASH_ITER(hh, hashTable, iter, tmp) {
        bucketsSize[iter->val]++;
        retSize += iter->val;
    }
    for (int i = 1; i <= maxFreq; i++) {
        buckets[i] = malloc(sizeof(char) * bucketsSize[i]);
    }
    memset(bucketsSize, 0, sizeof(bucketsSize));
    HASH_ITER(hh, hashTable, iter, tmp) {
        buckets[iter->val][bucketsSize[iter->val]++] = iter->key;
    }
    char* ret = malloc(sizeof(char) * (retSize + 1));
    retSize = 0;
    for (int i = maxFreq; i > 0; i--) {
        char* bucket = buckets[i];
        for (int j = 0; j < bucketsSize[i]; j++) {
            for (int k = 0; k < i; k++) {
                ret[retSize++] = bucket[j];
            }
        }
    }
    ret[retSize] = '\0';
    return ret;
}