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## Experts Tips to Solve NP-Complete Problems in GATE 2017

There are certain computational problems which cannot be solved by algorithms even with unlimited time complexity.

NP-Complete problems are another set of problems which fall under this category.

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## Classification of Problems

1. P :These are a set of problems which can be solved in polynomial time by a deterministic Turing machine.
1. NP : These are a set of decision problems which can be solved in Polynomial time by a Non-Deterministic Turing machine.

NP is a superset of P.

1. NP-Complete

These problems are the hardest problems in the NP set.

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A decision problem C is NP-Complete if

• C is in NP
• Every problem in NP is reducible to C in polynomial time

From the figure, it is clear that NP-Complete is a subset of NP-Hard set.

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## What is Reduction?

Let L1 and L2 denote two decision problems. Suppose algorithm A2 solves L2.

The main purpose is to find a transformation from L1 to L2  such that Algorithm A2 can be a part of A1 to solve L1.

## How to Prove that a given problem is NP-Complete?

Tips:

The technique is to take a known NP-complete problem and reduce it to L.

Then, we can prove that L is NP-Complete by transitivity of reduction if polynomial time reduction is possible.

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Solved Examples

Q1. Which of the following statements are TRUE?

1. The problem of identifying whether a cycle exists in an undirected graph is in P
2. The problem of identifying whether a cycle exists in an undirected graph is in NP
• If a problem A is NP-Complete, there exists a non-deterministic polynomial time algorithm to solve A

Options

1. 1, 2 and 3
2. 1 and 2 only
3. 2 and 3 only
4. 1 and 3 only

Solution

1. In order to find whether there is a cycle in an undirected graph, we can use either BFS or DFS. The time complexity of a DFS based implementation is O(V + E) which is a polynomial
2. A problem in P domain is definitely NP.
• This statement is also true.

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Q2.   Let S be an NP-complete problem and Q and R be two other problems not known to be in NP. Q is polynomial time reducible to S and S is polynomial-time reducible to R. Which one of the following statements is true?

Options

1. R is NP-complete
2. R is NP-hard
3. Q is NP-complete
4. Q is NP-hard

Solution:

Statement (A) is incorrect since R is not in NP.

Statement (B) is correct since S is polynomial time reducible to R

Statement (C) is incorrect since Q is not in NP

Statement (D) is incorrect since there is no NP-Complete problem which is polynomial time

Turing-reducible to Q.

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