Random Selection from a Linked List
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 :
- Use random number function rand() that returns a number between 0 and 1.
- It must be done in O(N) time
p.s. - For Remarks please hit the question name once, given in the question itself. For Hint, hit it twice and for the solution, hit it thrice.
Hints: No hints. For the solution, please hit the question name again.
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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[index];
}
}
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;
}
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