# Optimality and Reduction Of Algorithm with Examples

Optimality and reduction are essential concepts in the design and analysis of an algorithm. They both play a crucial role in the development of efficient algorithms that solve problems effectively.

## In Shorts

Optimality and Reduction are fundamental and important concepts in algorithm design. Optimality focuses on finding the best possible solution in terms of time and space complexity whereas Reduction aims to allow us to solve difficult problems by transforming them into easier ones that have known solutions. Both concepts help in the development of efficient algorithms for complex problems.

## 1. Optimality

In the context of algorithms, Optimality measures whether an algorithm is optimal or not for the provided problem. This is calculated in terms of time and space complexity. In another way, we can say that an algorithm for a given problem is optimal if its complexity reaches the lower bound over all the algorithms that are solving this problem.

For example,

Suppose there is an algorithm that is solving the problem “the intersection of n segments”. The solution for this problem will execute at least n^2 operations in the worst case even if it does nothing but print the output. So if the problem runs for n^2 times then in the worst case its complexity will be O(n^2).

If we find an algorithm that solves this similar problem in less time i.e. in Omega(n)^2, then we can say this one is optimal with complexity omega(n)^2.

### Optimality is categorized into two types

a). Time Optimality:

• An algorithm can be considered time-optimal if it solves a problem in the minimum possible time.
• It ensures that no other algorithm can solve this problem in lesser time.
• For example, let’s consider comparison-based sorting algorithms, such as Quicksort and Merge Sort. It’s time complexity is O(n log n), which means no other algo sort the array in lesser time and faster than Quicksort and Merge Sort. So Quicksort and Merge Sort sorting algo for this specific problem are optimal in terms of time.

b). Space Optimality:

• An algorithm is considered space-optimal if it uses the minimum possible amount of memory or space to solve a problem.
• Space optimality deals with memory size. This space could be anything like large input sizes or limited memory resources.
• For example, Suppose we have a problem to find the maximum element in an array. A space-optimal algorithm will be those that will not store the entire array. But It would keep track of the current maximum element while traversing the array, using only a constant amount of space.

## 2. Reduction

Reduction is a  process of converting an original problem to another problem that is known or solvable or has already solved solution. We choose this approach when the given problem is difficult to solve directly. But by reducing the complex problem into an easier solvable problem, that can be solved effectively.

Also, It can be used to prove that some problems are unsolvable if they can be reduced to a problem that has not been yet solved.

Here A is reduced to B and if B is solvable then A will also be solvable.

For Example,

As we know that a graph problem is solvable. So if any complex problem that can be transformed into a graph problem then that problem is also solvable.

A reduction is a powerful tool that allows us to utilize existing algorithms or problem-solving techniques to solve new or complex problems. With the help of this concept, we can reuse previous solutions to solve and develop of efficient algorithms.

### Reduction can be performed in two directions:

a). Reduction from problem A to problem B :

• This means that an instance of problem A can be transformed into an instance of problem B.
• Here solving problem B will provide the solution to problem A.
• For Example, the Traveling Salesman Problem (TSP) can be reduced to the Hamiltonian Cycle Problem (HCP). And If we find a Hamiltonian cycle in the graph we can also find the optimal solution for the TSP.

b). Reduction from problem B to problem A:

• This means that an instance of problem B can be transformed into an instance of problem A.
• Here solving problem A will provide the solution to problem B.
• For Example, the Minimum Spanning Tree (MST) problem can be reduced to the Shortest Path Problem (SPP). Where finding the shortest path in a weighted graph implies finding the minimum spanning tree.