CSES Solutions - Factory Machines

A factory has N machines which can be used to make products. Your goal is to make a total of T products.

For each machine, you know the number of seconds it needs to make a single product, given as array K[]. The machines can work simultaneously, and you can freely decide their schedule.

What is the shortest time needed to make T products?

Examples:

Input: N = 3, T = 7, K[] = {3, 2, 5}
Output: 8
Explanation:

• Machine 1 will make 2 products. Total time taken = 3 * 2 = 6 seconds.
• Machine 2 will make 4 products. Total time taken = 2 * 4 = 8 seconds.
• Machine 3 will make 1 product. Total time taken = 5 * 1 = 5 seconds.

Shortest time to make 7 products = 8 seconds.

Input: N = 2, T = 5, K[] = {1, 2}
Output: 4
Explanation:

• Machine 1 will make 3 products. Total time taken = 1 * 3 = 3 seconds.
• Machine 2 will make 2 products. Total time taken = 2 * 2 = 4 seconds

Shortest time to make 5 products = 4 seconds.

Approach: To solve the problem, follow the below idea:

The problem can be solved using Binary Search on Answer. We can binary search for the time needed to make the T products. If we observe carefully, the time to make T products will be at least 1 second and the maximum time to make T products will be when we have only 1 machine and it takes maximum amount of time to make one product. So, we can initialize low = 1 and high = T * min value in K[]. Now, we can find mid and find the total number of products we can making using all machines. If the total number of products is less than T, then it means we need more time so we will shift lo = mid + 1. Otherwise, we shift high = mid - 1 and update the answer. We keep on searching till low <= high and after all the iterations, we can return the final answer.

Step-by-step algorithm:

• Maintain a function check(mid, N, T, K) to check whether it is possible to make T products in time <= mid.
• Also maintain a variable ans to store the minimum time to make T products.
• Declare the range in which our answer can lie, that is low = 1, high = T * min value in K[].
• Iterate till low <= high,
  • Find the mid in range [low, high] and check if we can make T products in time <= mid.
  • If we can make T products in time <= mid, then update the answer and reduce search space by moving high to mid - 1.
  • Otherwise, move low = mid + 1.
• After all the iterations, return ans as the final answer.

Below is the implementation of the algorithm:

8

Time Complexity: O(N * log(T * max(K[]))), N is the number of machines, T is the number of products and K[] is the array of time which each machine takes to make 1 product.
Auxiliary Space: O(1)