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CSC 413/513: Intro to Algorithms

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CSC 413/513: Intro to Algorithms NP Completeness Problems and Instances Problem: Sort(A) Problem instance: Sort([4,2,7,1,6,9]) Input encoding Bin(4) concat Bin(2 ... – PowerPoint PPT presentation

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Title: CSC 413/513: Intro to Algorithms


1
CSC 413/513 Intro to Algorithms
  • NP Completeness

2
Problems and Instances
  • Problem Sort(A)
  • Problem instance Sort(4,2,7,1,6,9)
  • Input encoding
  • Bin(4) concat Bin(2) concat Bin(7)
  • Size n lg x
  • n size of A
  • lg x max. number of bits required to represent
    Ai, for any i
  • We say size ?(n), or simply n

3
NP-Completeness
  • Some problems are intractable as the input size
    increases, we are unable to solve them in
    reasonable time
  • What constitutes reasonable time? Standard
    working definition polynomial time
  • On an input of size n the worst-case running time
    is O(nk) for some constant k
  • Polynomial time O(n2), O(n3), O(1), O(n lg n)
  • Not in polynomial time O(2n), O(nn), O(n!)

4
Polynomial-Time Problems
  • Are some problems solvable in polynomial time?
  • Of course every algorithm weve studied provides
    polynomial-time solution to some problem
  • Except DP algorithm to 0-1 knapsack
  • We define P to be the class of problems solvable
    in polynomial time
  • That is, all instances of such problems are
    polynomially solvable (e.g., sorting)
  • These are the easy problems

5
Polynomial-Time Problems
  • Are all problems solvable in polynomial time?
  • No Turings Halting Problem is not solvable by
    any computer, no matter how much time is given
  • But we often detect infinite loops in a program
    by inspection, and hence solve the halting
    problem
  • However, there are instances that are not
    solvable, even by our brains
  • Such problems are clearly intractable, not in P
  • Similarly, there are problems that can be solved,
    but we havent found algorithms that will take
    any instance of the problem and solve it in
    polynomial time

6
NP-Complete Problems
  • The NP-Complete problems are an interesting class
    of problems whose status is unknown
  • No polynomial-time algorithm has been discovered
    for an NP-Complete problem, for all instances of
    it
  • No suprapolynomial lower bound has been proved
    for some instance of any NP-Complete problem,
    either
  • We call this the P NP question
  • The biggest open problem in CS

7
An NP-Complete Problem Hamiltonian Cycles
  • An example of an NP-Complete problem
  • A hamiltonian cycle of an undirected graph is a
    simple cycle that contains every vertex
  • The hamiltonian-cycle problem given a graph G,
    does it have a hamiltonian cycle?

Describe a naïve algorithm for solving the
hamiltonian-cycle problem. Running time?
8
P and NP
  • As mentioned, P is set of problems that can be
    solved in polynomial time
  • NP (nondeterministic polynomial time) is the set
    of problems that can be solved in polynomial time
    by a nondeterministic computer
  • What the hell is that?

9
Nondeterminism
  • Think of a non-deterministic computer as a
    computer that magically guesses a solution, and
    then has to verify that it is correct
  • If a solution exists, n.d. computer always
    guesses it
  • One way to imagine it a parallel computer that
    can freely spawn an infinite number of processes
  • Have one processor work on each possible solution
  • All processors attempt to verify that their
    solution works, in parallel
  • If a processor finds it has a working solution
  • So NP problems verifiable in polynomial time
  • Note P NP (think of verifying the output of
    sorting)

10
P and NP
  • Summary so far
  • P problems that can be solved in polynomial
    time
  • NP problems for which a solution can be
    verified in polynomial time
  • P NP
  • Unknown whether P NP (most suspect not)
  • Hamiltonian-cycle problem is in NP
  • Dont know how to solve in polynomial time
  • But, easy to verify solution in polynomial time
    (How?)

11
Intuition behind NP-Complete problems
  • Many problems exhibit structure that is exploited
  • E.g., recursive structure in sorting, optimal
    substructure in problems solved by DP, greedy
    etc.
  • But, some problems seem to have no exploitable
    structure
  • E.g., longest simple path in a graph
  • Contrast with shortest paths in graphs, which has
    the optimal substructure
  • Typically, these are the NP-Complete problems

Does not have optimal substructure
12
What about 0-1 Knapsack?
  • Has optimal substructure
  • But input contains a number (W), and the optimal
    substructure depends on the magnitude of this
    number, not its size
  • Algorithm turns out to be exponential in the size
    of W (although polynomial in the magnitude of W)
  • Its DP Algorithm is called pseudo-polynomial
  • 0-1 knapsack is called weakly NP-complete

13
NP-Complete Problems
  • We will see that NP-Complete problems are the
    hardest problems in NP
  • If any one NP-Complete problem can be solved in
    polynomial time
  • then every NP-Complete problem can be solved in
    polynomial time
  • and in fact every problem in NP can be solved in
    polynomial time (which would show P NP)
  • Thus solve hamiltonian-cycle in O(n100) time,
    youve proved that P NP. Retire rich famous.

14
A Simplification
  • Henceforth, we will only talk about decision
    problems, with boolean answers
  • Most problems of interest can be posed as
    decision problems
  • E.g., SHORTEST_PATH(G,u,v)
  • Its decision problem is PATH(G,u,v,k) is there a
    path between u and v involving no more than k
    edges?
  • Can solve SHORTEST_PATH by calling PATH
    repeatedly with decreasing k, until the answer is
    No
  • So, solve decide

15
How to Verify?
  • How do you verify the answer to a decision
    problem (in polynomial time)?
  • Need a certificate
  • E.g., certificate for PATH(G,u,v,k) would be the
    path ltu,v1,v2,,vgt
  • How do you verify the certificate?
  • Check length k
  • Check first vertex is u, last vertex is v
  • Check all path-edges actually exist
  • How much time?

16
Why only Decision Problems?
  • So that problems can be considered as languages
  • Then, algorithm Turing machine
  • Consider the binary encoding of a problem
    instance (i.e., the input)
  • Collect all input binary strings for which the
    answer/output is Yes
  • This set is a language
  • A machine that decides the language also solves
    the problem
  • So, decision problem language

17
Reduction
  • The crux of NP-Completeness is reducibility
  • Informally, a problem P can be reduced to another
    problem Q if any instance of P can be easily
    rephrased as an instance of Q, the solution to
    which provides a solution to the instance of P
  • What do you suppose easily means?
  • This rephrasing is called transformation
  • Intuitively If P reduces to Q, P is no harder
    to solve than Q

Polynomial time
18
Polynomial Reduction
  • Polynomial reduction can be used for polynomial
    time solution
  • If A polynomially reduces to B, and we know a
    polynomial algorithm to B, then we can solve A in
    polynomial time as well.

19
Reducibility
  • An example
  • A Given a set of Booleans, is at least one TRUE?
  • B Given a set of integers, is their sum gt 0?
  • Transformation Given (x1, x2, , xn) construct
    (y1, y2, , yn) where yi 1 if xi TRUE, yi 0
    if xi FALSE
  • Another example
  • Solving linear equations is reducible to solving
    quadratic equations
  • How can we easily use a quadratic-equation solver
    to solve linear equations?

20
NP-Hard and NP-Complete
  • If P is polynomial-time reducible to Q, we denote
    this P ?p Q
  • Definition of NP-Hard and NP-Complete
  • If all problems R ? NP are reducible to P, then P
    is NP-Hard
  • We say P is NP-Complete if P is NP-Hard and P ?
    NP
  • If P ?p Q and P is NP-Complete, Q is also NP-
    Complete

21
Why Prove NP-Completeness?
  • Though nobody has proven that P ! NP, if you
    prove a problem NP-Complete, most people accept
    that it is probably intractable
  • Therefore it can be important to prove that a
    problem is NP-Complete
  • Dont need to come up with an efficient algorithm
  • Can instead work on approximation algorithms

22
Proving NP-Completeness
  • What steps do we have to take to prove a problem
    P is NP-Complete?
  • Pick a known NP-Complete problem Q
  • Reduce Q to P
  • Describe a transformation that maps an arbitrary
    instance of Q to some instance of P, s.t. yes
    for P yes for Q
  • Prove the transformation works
  • Prove the transformation takes polynomial time
  • Oh yeah, prove P ? NP (What if you cant?)

23
Coming Up
  • Given one NP-Complete problem, we can prove many
    interesting problems NP-Complete
  • Graph coloring (recall the wrestler-rivalry HW
    problem)
  • Hamiltonian cycle
  • Hamiltonian path
  • Knapsack problem
  • Traveling salesman
  • Job scheduling with penalties
  • Many, many more
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