Problem : RRSCHED

Difficulty : Medium

Prerequisites : BIT

Explanation :

A very naive approach would be to iterate over time and keep on decreasing the no of tasks to be performed and storing the task completion time in a separate array.This implementation would definitely give TLE O(N*T) with the given constraints.

In the above approach we can observe that we can do better by jumping times from t = completion of easiest task (w.r.t time) to t = completion of next easiest task .

For this we need to sort the input array w.r.t time .But we would also want their original indices(because order matters).So you can use pair<int,int> to store both the index and time of a task.That way you would not lose your index’s by sorting the array.

In the following discussion:

Pair [i] denotes i th pair in sorted array Pair[i].Time denotes time needed for ‘Pir[i].Index’ th task to complete.

Now for every iteration all we need to do is to keep track of Current time and do as follows.

  1. Time Taken for present task to be completed = (no of remaining elements)*(Pair[i].Time-Pair[i-1].Time) + 1*no of Tasks before Pair[i].Index which are not completed

  2. Extra Time to complete the round = 1*no of Tasks after Pair[i].Index which are not completed.

  3. Update Pair[i].Index as completed

Thus BIT comes in handy here where we have to query the no of tasks not completed and update a task as completed in O(logN). For this purpose initially keep a BIT array and update every element by ‘+1’ which means that the task is not completed yet.You can use Update(i,-1) to update the task as completed. You can find the no of tasks which are not completed before an index i by Query(i).

The pesudo code is as follows:

//Initialize bit array to zero 

BIT[MaxN] = 0

// Update bit array 

Upadte(i , x)
	for(; i <= n; i += i&-i)
    	BIT[i] += x

//Query Cummulative Frequency in a bit array 

	for(; i>0 ; i-=i&-i)
    	s += BIT[i]
	return s

//Take Input and Do the necessary initializations for BIT array

for(i = 1; i <= N; i++)
	Update(i, 1)

//Main Part

CurrentTime = 0
for(i = 1; i <= N; i++)

	//N-i+1 total uncompleted tasks
	CurrentTime += (Pair[i].Time - Pair[i-1].Time - 1)*(N-i+1)
	//query for uncompleted tasks before the present task
	CurrentTime +=  Query(Pair[i].Index)
	//Store the final anwer for the present task
	Answer[Pair[i].Index] = CurrentTime
	//Update current task as completed
	Update(Pair[i].Index, -1)
	//Increase by no of uncompleted tasks after the present task
	CurrentTime += (N-i) - Query(Pair[i].Index)

//Note that the above psudo code also handles the case where two tasks have equal completion Time.