Optimized Longest Path is NP Complete
Optimized Longest Path Problem: The optimized longest path problem states that given a graph G, of a set of vertices V and edges E, the task is to prove that there is a path of length at least K between a set of nodes V_{s} and V_{e}.
Problem Statement: Given a graph G(V, E, K) and a set of nodes V_{s} and V_{e} with the sequence of nodes, of length ≥ K.
Explanation:
An instance of the problem is an input specified to the problem. An instance of the optimizedlongest path problem is G(V, E, V_{s}, V_{e}, K). Since an NPcomplete problem is a problem which is both in NP and NPHard, the proof for the statement that a problem is NPComplete consists of two parts:
 The problem itself is in NP class.
 All other problems in NP class can be polynomialtime reducible to that.
(B is polynomialtime reducible to C is denoted as B≤P^{C})
If the 2nd condition is only satisfied then the problem is called NPHard.
But it is not possible to reduce every NP problem into another NP problem to show its NPCompleteness all the time. That is why if we want to show a problem is NPComplete we just show that the problem is in NP and any NPComplete problem is reducible to that then we are done, i.e. if B is NPComplete and B ≤ P^{C }For C in NP, then C is NPComplete. Thus, we can verify that the Optimized Longest Path Problem is NPComplete using the following two propositions:

Optimizedlongest Path Problem is in NP:
If any problem is in NP, given a ‘certificate’, which is a solution to the problem and an instance of the problem then it can be verified (check whether the solution given is correct or not) that the certificate in polynomial time. This can be done by a path P, consisting of a set of vertices < V_{1}, V_{2}, V_{3}, ….V_{n} >. Verify if the path connects V_{1}, and V_{n} completely and the length of the path is at most K. 
Optimizedlongest Path Problem is NPHard:
In order to prove that the Longest Path is NPHard, deduce a reduction from a known NPHard to the problem. Carry out a reduction in which an undirected Hamiltonian Path problem can be reduced to the Longest Path problem. The undirected hamiltonian path uses the input a graph G(V_{1}, V_{n}) where graph G has nodes V_{1} and V_{n}. Undirected Hamiltonian Path is an Undirected path along the graph starting at one vertex and ending at another traversing all nodes.Now, let K be the numbers of nodes in G. Every instance of Undirected Hamiltonian Path can be converted to the Longest Path in the following way:
For input G(V_{1}, V_{n}), output G(V_{1}, V_{n}, k). This reduction takes polynomial time by simply counting the number of vertices in G. The reduction can be proved by the following two propositions: Assume the original graph G(V, E) is provided with nodes V_{1} and V_{n} has an undirected Hamiltonian path, which traverses all the vertices, therefore G(V, E, K) is true because any two nodes in G will be connected by a path of length equal to its nodes i.e., K therefore Longest Path problem holds.
 Let us assume the graph G'(V, E, V_{s}, V_{e}, K) has a Lpath of length K from V_{s} to V_{e}, which implies G’ contains a simple path of length K from V_{s} to V_{e}.
But, G contains K vertices, hence traverses all vertices starting at V_{s} and ending at V_{e} forming a hamiltonian path, G'(V_{s}, V_{e}).Let V_{1} ≡ B and V_{n} ≡ D
Now, G has an Undirected Hamiltonian Path ≡ BCAD of K = 4.
Therefore, G contains an optimized path of length = 4 between B and D.