Top k most frequent elements in a stream

#1

I am not able to implement the logic for this question. Please help

#2

Let’s understand this problem with the help of two examples:
1.
arr[] = {5, 2, 1, 3, 2}
k = 4
Output :
5 2 5 1 2 5 1 2 3 5 2 1 3 5
Explanation:
Given array is arr[] = {5, 2, 1, 3, 2} and k = 4
Step 1:After reading 5, there is only one element 5 whose frequency is max till now. so print 5.
Step 2:After reading 2, we will have two elements 2 and 5 with same frequency. As 2, is smaller than 5 but their frequency is same so we will print 2 5.
Step 3: After reading 1, we will have 3 elements 1,2 and 5 with same frequency, so print 1 2 5.
Step 4: Similarly after reading 3, print 1 2 3 5
Step 5: After reading last element 2, since 2 has already occurred so we have now frequency of 2 as 2. So we keep 2 at the top and then rest of element with same frequency in sorted order. So print, 2 1 3 5.

arr[] = {5, 2, 1, 3, 4}
k = 4
Output :
5 2 5 1 2 5 1 2 3 5 1 2 3 4
Explanation:
Given array is arr[] = {5, 2, 1, 3, 4} and k = 4
Step 1:After reading 5, there is only one element 5 whose frequency is max till now. so print 5.
Step 2:After reading 2, we will have two elements 2 and 5 with same frequency. As 2, is smaller than 5 but their frequency is same so we will print 2 5.
Step 3: After reading 1, we will have 3 elements 1,2 and 5 with same frequency, so print 1 2 5.
Step 4: Similarly after reading 3, print 1 2 3 5
Step 5: After reading last element 4, since 4 is smaller than 5. So we keep 4 at the top and then rest of element with same frequency in sorted order and will discard 5 as k=4. So print, 2 1 3 4.

Approach:

Iterate through the array which contains stream of numbers.
To keep track of top k elements, make a top vector of size k+1.
For every element in the stream increase its frequency and store it in the last position of top vector. We can use hashing for efficiently fetching frequency of an element and increasing it.
Now find the position of element in top vector and iterate from that position to zero. For finding position we can make use of the find() function in C++ STL, it returns an iterator pointing to element if found in the vector.
And make that list of k+1 numbers sorted according to frequency and their value.
Print top k elements form top vector.
Repeat the above steps for every element in the stream.

#3

Please mark your doubt as resolved if you are satisfied

#4

how to implement this using heaps

#5

You can refer to this link here if you wish to solve this using max heap

#6

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