TheAlgorithms-C/leetcode/src/442.c

int cmpval (const void *a, const void *b) {
    return *(int *)a - *(int *)b;
}

int* findDuplicates(int* nums, int numsSize, int* returnSize){

    int i;
    qsort(nums, numsSize, sizeof(int), cmpval);
    int *retArr = malloc(numsSize * sizeof(int));
    *returnSize = 0;
    for (i = 0; i < numsSize - 1;) {
        if (nums[i] == nums[i + 1]) {\
            retArr[*returnSize] = nums[i];
            *returnSize = *returnSize + 1;
            i = i + 2;
        } else {
            i = i + 1;
        }
    }
    return retArr;
}