Random selection

There is a linked list of numbers of length N. N is very large and you don’t know N. You have to write a function that will return k random numbers from the list. Numbers should be completely random.
Conditions :

1. Use random number function rand() that returns a number between 0 and 1
2. It must be done in O(N) time

Solution :

1. Take 2 arrays R[K] and Num[K]
2. For each element in sorted array read one by one and do :
1. Generate a random number r
2. If R is not full, insert r into R and element into Num array else replace r with highest random number in R and insert current element into corresponding index in Num array (Note that we want to keep the least K random numbers and corresponding elements in list)
3. Print Num[]

Complexities :

• Time : O(N)
• Space : O(K)

Code :

void random_selection(int[] input, int N, int K){

int number[K];
double random[K];
int count = 0;

for(int i=0; i<N; i++){

double r = rand();

    if(r < number[count-1]){

index = insert(random, K, &count, r);

number[index] = input[i];

              }

}

for(int i=0; i<K; i++){

printf("%d ", number[i]);

}

}

int insert(double *list, int K, int *count, double entry){

if(*count == K-1){

(*count)--;

}

int i = *count;
while(*(list+i) > entry){

*(list + i + 1) = *(list + 1);
i--;

}

*(list + i + 1) = entry;
(*count)++;

return i+1;

}