题目：

题解：

int comp(const void* a, const void* b) {return *(int*)a - *(int*)b;
}int** fourSum(int* nums, int numsSize, int target, int* returnSize, int** returnColumnSizes) {int** quadruplets = malloc(sizeof(int*) * 1001);*returnSize = 0;*returnColumnSizes = malloc(sizeof(int) * 1001);if (numsSize < 4) {return quadruplets;}qsort(nums, numsSize, sizeof(int), comp);int length = numsSize;for (int i = 0; i < length - 3; i++) {if (i > 0 && nums[i] == nums[i - 1]) {continue;}if ((long) nums[i] + nums[i + 1] + nums[i + 2] + nums[i + 3] > target) {break;}if ((long) nums[i] + nums[length - 3] + nums[length - 2] + nums[length - 1] < target) {continue;}for (int j = i + 1; j < length - 2; j++) {if (j > i + 1 && nums[j] == nums[j - 1]) {continue;}if ((long) nums[i] + nums[j] + nums[j + 1] + nums[j + 2] > target) {break;}if ((long) nums[i] + nums[j] + nums[length - 2] + nums[length - 1] < target) {continue;}int left = j + 1, right = length - 1;while (left < right) {long sum = (long) nums[i] + nums[j] + nums[left] + nums[right];if (sum == target) {int* tmp = malloc(sizeof(int) * 4);tmp[0] = nums[i], tmp[1] = nums[j], tmp[2] = nums[left], tmp[3] = nums[right];(*returnColumnSizes)[(*returnSize)] = 4;quadruplets[(*returnSize)++] = tmp;while (left < right && nums[left] == nums[left + 1]) {left++;}left++;while (left < right && nums[right] == nums[right - 1]) {right--;}right--;} else if (sum < target) {left++;} else {right--;}}}}return quadruplets;
}