代码实现：

快排 + 哈希表 ——超时

/**
 * Note: The returned array must be malloced, assume caller calls free().
 */
void swap(int *m, int *n) {
    int temp = *m;
    *m = *n;
    *n = temp;
}

// 快排
void sort(int *a, int l, int r) { // 左闭右开
    if (r - l <= 2) {
        if (r - l <= 1) {
            return;
        } 
        if (a[l] > a[r - 1]) {
            swap(&a[l], &a[r - 1]);
        }
        return;
    }
    int i = l, j = r - 1;
    int pivot = a[i];
    while (i < j) {
        while (i < j && a[j] >= pivot) {
            j--;
        }
        if (i < j) {
            a[i] = a[j];
            i++;
        }
        while (i < j && a[i] <= pivot) {
            i++;
        }
        if (i < j) {
            a[j] = a[i];
            j--;
        }
    }
    a[i] = pivot;
    sort(a, l, i);
    sort(a, i + 1, r);
}

int* findOriginalArray(int *changed, int changedSize, int *returnSize) {
    *returnSize = 0;
    if (changedSize == 0 || changedSize % 2) {
        return NULL;
    }
    int *res = malloc(sizeof(int) * changedSize / 2);
    sort(changed, 0, changedSize);
    int hash[100001] = {0};
    for (int i = 0; i < changedSize; i++) {
        hash[changed[i]]++;
    }
    for (int i = 0; i < changedSize; i++) {
        if (hash[changed[i]] == 0) {
            continue;
        }
        if (changed[i] == 0 && hash[0] < 2) { // 特别处理0的情况
            *returnSize = 0;
            return NULL;
        }
        if (hash[changed[i] * 2] == 0) {
            *returnSize = 0;
            return res;
        }
        res[(*returnSize)++] = changed[i];
        hash[changed[i]]--;
        hash[changed[i] * 2]--;
    }
    return res;
}