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;
}