static int compare(void *a, void *b)

{

    return *(int *) a - *(int *) b;

}

​

#define MALLOC_UNIT 5000

/**

 * Return an array of arrays of size *returnSize.

 * The sizes of the arrays are returned as *returnColumnSizes array.

 * Note: Both returned array and *columnSizes array must be malloced, assume

 * caller calls free().

 */

int **threeSum(int *nums,

               int numsSize,

               int *returnSize,

               int **returnColumnSizes)

{

    *returnSize = 0;

    if (numsSize < 3)

        return NULL;

​

    qsort(nums, numsSize, sizeof(int), compare);

    int count = MALLOC_UNIT, ret_sz = 0;

    int **ret = (int **) malloc(sizeof(int *) * count);

    *returnColumnSizes = malloc(sizeof(int) * count);

    for (int i = 0; i < numsSize - 2; i++) {

        /* ignore the duplicate target */

        if (i > 0 && nums[i] == nums[i - 1])

            continue;

​

        int target = (-1) * (nums[i]), left = i + 1, right = numsSize - 1;

        while (left < right) {

            if (left > i + 1 && nums[left - 1] == nums[left]) {

                left++;

                continue;

            }

​

            if (nums[left] + nums[right] == target) {

                ret_sz++;

                if (ret_sz == count) {

                    count <<= 1;

                    ret = realloc(ret, sizeof(int *) * count);

                    *returnColumnSizes =

                        realloc(*returnColumnSizes, sizeof(int) * count);

                }

                ret[ret_sz - 1] = malloc(sizeof(int) * 3);

                ret[ret_sz - 1][0] = nums[i];

                ret[ret_sz - 1][1] = nums[left];

                ret[ret_sz - 1][2] = nums[right];

                (*returnColumnSizes)[ret_sz - 1] = 3;

                left++;

            } else if (nums[left] + nums[right] < target) {

                left++;

            } else {

                /* nums[left] + nums[right] > target */

                right--;

            }

        }

    }

    *returnSize = ret_sz;

    return ret;

}