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Minimum Waiting Time

Minimize the total waiting time for a set of queries with mentioned strategies for efficient scheduling.

Minimum Waiting Time
Greedy
easy

Published At

6/4/2021

Reading Time

~ 3 min read

You're given a non-empty array of positive integers representing the amounts of time that specific queries take to execute. Only one query can be executed at a time, but the queries can be executed in any order.

A query's waiting time is defined as the amount of time that it must wait before its execution starts. In other words, if a query is executed second, then its waiting time is the duration of the first query; if a query is executed third, then its waiting time is the sum of the durations of the first two queries.

Write a function that returns the minimum amount of total waiting time for all of the queries. For example, if you're given the queries of durations [1, 4, 5], then the total waiting time if the queries were executed in the order of [5, 1, 4] would be (0) + (5) + (5 + 1) = 11. The first query of duration 5 would be executed immediately, so its waiting time would be 0, the second query of duration 1 would have to wait 5 seconds (the duration of the first query) to be executed, and the last query would have to wait the duration of the first two queries before being executed.

Note: you're allowed to mutate the input array.

Sample Input

jsqueries = [3, 2, 1, 2, 6]
jsqueries = [3, 2, 1, 2, 6]

Sample Output

js17
js17

Hints

Hint 1

Even though you don't need to return the actual order in which the queries will be executed to minimize the total waiting time, it's important to determine what this order should be. Start by doing so.

Hint 2

Can you solve this problem with constant space? What advantage does being able to mutate the input array provide?

Hint 3

Sort the input array in place, and execute the shortest queries in their sorted order. This should allow you to calculate the minimum waiting time.

Hint 4

Create a variable to store the total waiting time, and iterate through the sorted input array. At each iteration, multiply the number of queries left by the duration of the current query, and add that to the total waiting time.

Optimal Space & Time Complexity

O(nlogn) time | O(1) space - where n is the number of queries

Solution-1
js// O(nlogn) + O(n)
// O(nlogn) time complexity
// space = O(1)
function minimumWaitingTime(queries) {
	queries = queries.sort((a, b) => a - b); // nlogn - time taken by sorting algorithm
	let total = 0
 
	for (let i = 0; i < queries.length; i++) { // n for the loop on queries
		total += queries[i] * (queries.length - (i + 1))
	}
  return total;
}
Solution-1
js// O(nlogn) + O(n)
// O(nlogn) time complexity
// space = O(1)
function minimumWaitingTime(queries) {
	queries = queries.sort((a, b) => a - b); // nlogn - time taken by sorting algorithm
	let total = 0
 
	for (let i = 0; i < queries.length; i++) { // n for the loop on queries
		total += queries[i] * (queries.length - (i + 1))
	}
  return total;
}

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Have a wonderful day.
Abhishek ūüôŹ

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