题目：

题解：

struct hashTable {int key;int val;UT_hash_handle hh;
};int fourSumCount(int* A, int ASize, int* B, int BSize, int* C, int CSize, int* D, int DSize) {struct hashTable* hashtable = NULL;for (int i = 0; i < ASize; ++i) {for (int j = 0; j < BSize; ++j) {int ikey = A[i] + B[j];struct hashTable* tmp;HASH_FIND_INT(hashtable, &ikey, tmp);if (tmp == NULL) {struct hashTable* tmp = malloc(sizeof(struct hashTable));tmp->key = ikey, tmp->val = 1;HASH_ADD_INT(hashtable, key, tmp);} else {tmp->val++;}}}int ans = 0;for (int i = 0; i < CSize; ++i) {for (int j = 0; j < DSize; ++j) {int ikey = -C[i] - D[j];struct hashTable* tmp;HASH_FIND_INT(hashtable, &ikey, tmp);if (tmp != NULL) {ans += tmp->val;}}}return ans;
}