**Prerequiste:** NP-Completeness

**NP Problem:**

The NP problems set of problems whose solutions are hard to find but easy to verify and are solved by Non-Deterministic Machine in polynomial time.

**NP-Hard Problem:**

Any decision problem P_{i} is called NP-Hard if and only if every problem of NP(say P<subj) is reducible to P_{i} in polynomial time.

**NP-Complete Problem:** Any problem is NP-Complete if it is a part of both NP and NP-Hard Problem.

** Difference between NP-Hard and NP-Complete**:

NP-hard | NP-Complete |
---|---|

NP-Hard problems(say X) can be solved if and only if there is a NP-Complete problem(say Y) can be reducible into X in polynomial time. | NP-Complete problems can be solved by deterministic algorithm in polynomial time. |

To solve this problem, it must be a NP problem. | To solve this problem, it must be both NP and NP-hard problem. |

It is not a Decision problem. | It is exclusively Decision problem . |

Example: Halting problem, Vertex cover problem, Circuit-satisfiability problem, etc. | Example: Determine whether a graph has a Hamiltonian cycle, Determine whether a Boolean formula is satisfiable or not, etc. |

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