Algorithm chapter 10

P, NP and NP-Complete
Problems

Introduction
An algorithm with O(n3) complexity isn’t bad
because it can still be run for fairly large
inputs in a reasonable amount of time.
In this chapter, we are concerned with
problems with exponential complexity.
Tractable and intractable problems
Study the class of problems for which no
reasonably fast algorithms have been found,
but no one can prove that fast algorithms do
not exist.
2

Our Old List of Problems
Sorting
Searching
Shortest paths in a graph
Minimum spanning tree
Traveling salesman problem
Knapsack problem
Towers of Hanoi
3

Tractability
An algorithm solves the problem in
polynomial time if its worst-case time
efficiency belongs to O(p(n)) where p(n) is a
polynomial of the problem’s input size n.

Problems that can be solved in polynomial
time are called tractable.

Problems that cannot be solved in polynomial
time are called intractable.
4

Classifying a Problem’s Complexity
Is there a polynomial-time algorithm that solves the
problem?
Possible answers:
yes
no
because it can be proved that all algorithms take
exponential time
because it can be proved that no algorithm exists at all
to solve this problem
don’t know, but if such algorithms were to be found, then
it would provide a means of solving many other problems
in polynomial time
5

Types of Problems
Optimization problem: construct a solution that
maximizes or minimizes some objective function
Decision problem: A question that has two
possible answers, yes and no.
Example: Hamiltonian circles: A Hamiltonian circle in
an undirected graph is a simple circle that passes
through every vertex exactly once. The decision
problem is: Does a given undirected graph have a
Hamiltonian circle?

6

Some More Problems (1)
Many problems will have decision and optimization versions
Traveling salesman problem
Optimization problem: Given a weighted graph, find Hamiltonian cycle of
minimum weight.
Decision problem: Given a weighted graph and an integer k, is there a
Hamiltonian cycle with total weight at most k?

Knapsack : Suppose we have a knapsack of capacity W (a positive
integer) and n objects with weights w1, …, wn, and values v1, …, vn
(where w1, …, wn, and v1, …, vn are positive integers)
Optimization problem: Find the largest total value of any subset of the
objects that fits in the knapsack (and find a subset that achieves the
maximum value)
Decision problem: Given k, is there a subset of the objects that fits in the
knapsack and has total value at least k?
7

Some More Problems (2)
Bin packing: Suppose we have an unlimited number of bins each of
capacity one, and n objects with sizes s1, …, sn, where 0 ≤ si≤ 1( si
are rational numbers)
Optimization problem: Determine the smallest number of bins into which
the objects can be packed(and find an optimal packing).
Decision problem: Given, in addition to the inputs described, an integer
k, do the objects fit in k bins?

Graph coloring:
coloring: assign a color to each vertex so that adjacent vertices are not
assigned the same color.
Chromatic number: the smallest number of colors needed to color G.
We are given an undirected graph G = (V, E) to be colored.
Optimization problem: Given G, determine the chromatic number .
Decision problem: Given G and a positive integer k, is there a coloring of
G using at most k colors? If so, G is said to be k-colorable. 8

The class P
P : the class of decision problems that can be solved in
O(p(n)), where p(n) is a polynomial on n.

Why use the existence of a polynomial time bound as the criterion?
if not, very inefficient
nice closure properties Given a computer program
machine independent in a strong sense and an input to it,
determine whether the
program will halt on that
What is the solvability of a decision problem?
input or continue working
Solvable/decidable in polynomial time indefinitely on it.
Solvable/decidable but intractable
Unsolvable/undecidable problems: e.g., the halting problem
No polynomial algorithm has been found, nor has the impossibility of
such an algorithm been proved. 9

The class NP
Informally, NP is the class of decision problems for which a given proposed
solution for a given input can be checked quickly(in polynomial time) to
see if it really is a solution.

Formally, NP: the class of decision problems that can be solved by
nondeterministic polynomial (NP) algorithms

A nondeterministic algorithm: a two-stage procedure that takes as its input
an instance I of a decision problem and does the following
“guessing” stage: An arbitrary string S is generated that can be
thought of as a guess at a solution for the given instance (but may be
complete gibberish as well)
“verification” stage: A deterministic algorithm takes both I and S as its
input and check if S is a solution to instance I, (outputs yes if s is a
solution and outputs no or not halt at all otherwise)

10

The class NP
A nondeterministic algorithm solves a decision problem if and
only if for every yes instance of the problem it returns yes on
some execution.

A nondeterministic algorithm is said to be polynomially
bounded if there is a polynomial p such that for each input of
size n for which the answer is yes, there is some execution of
the algorithm that produces a yes output in at most p(n) steps.

11

Example: graph coloring
Nondeterministic graph coloring
First phase: generate a string s, a list of
characters, c1c2…cq ,which the second
phase interpret as a proposed coloring
solution
Second phase: interpret the above
characters as colors to be assigned to the
vertices ci vi

12

Example: CNF Satisfiability
The problem: Given a boolean expression expressed in
conjunctive normal form(CNF), can we assign values
true and false to variables to satisfy the CNF ?
This problem is in NP. Nondeterministic algorithm:
Guess truth assignment Make the formula true
Check assignment to see if it satisfies CNF formula

Example:
(A⋁¬B ⋁ ¬C ) ⋀ (¬A ⋁ B) ⋀ (¬ B ⋁ D ⋁ E ) ⋀ (F ⋁ ¬ D)

Truth assignments:
A B CD E F
1. 0 1 1 0 1 0 literals
2. 1 0 0 0 0 1
3. 1 1 0 0 0 1
4. … 13

The Relationship between P & NP (1)
P ⊆ NP
(I ∈ P I ∈ NP): Every decision problem solvable
by a polynomial time deterministic algorithm is
also solvable by a polynomial time
nondeterministic algorithm.
To see this, observe that any deterministic
algorithm can be used as the checking stage of a
nondeterministic algorithm.
If I ∈ P, and A is any polynomial deterministic algorithm
for I, we can obtain a polynomial nondeterministic
algorithm for I merely by using A as the checking stage
and ignoring the guess. Thus I ∈ P implies I ∈ NP

14

The Relationship between P & NP (2)
?
P = NP (NP ⊆ P?)
Can the problems in NP be solved in
polynomial time?
A tentative view
NP

P

15

NP-Completeness (1)
NP-completeness is the term used to describe decision
problems that are the hardest ones in NP
If there were a polynomial bounded algorithm for an NP-
complete problem, then there would be a polynomial bounded
algorithm for each problem in NP.
Examples
Hamiltonian cycle
Traveling salesman
Knapsack
Bin packing
Graph coloring
Satisfiability

16

Informal Definition of NP-Completeness
Informally, an NP-complete problem is a problem in
NP that is as difficult as any other problem in this
class, because by definition, any other problem in NP
can be reduced to it in polynomial time.
A decision problem D1 is said to be polynomially
reducible to a decision problem D2 if there exists a
function t that transforms instances of D1 to instances
of D2 such that
1. t maps all yes instances of D1 to yes instances of D2 and
all no instances of D1 to all no instances of D2.
2. t is computable by a polynomial-time algorithm.

If D2 is polynomially solvable, then D1 can also be solved in polynomial time.

17

An Example of Polynomial Reductions
Problem P: Given a sequence of Boolean
values, does at least one of them have the
value true?
Problem Q: Given a sequence of integers, is
the maximum of the integers positive?
Transformation T:
t(x1, x2, …, xn) = (y1, y2, …, yn), where
yi = 1 if xi = true, and yi = 0 if xi = false.

18

Another Example of Polynomial Reductions

For example
a Hamiltonian circuit problem is polynomially reducible to the
decision version of the traveling salesman problem.
Hamiltonian circuit problem(HCP): Does a given undirected graph have
a Hamiltonian cycle?
Traveling salesman problem(TSP): Given a weighted graph and an
integer k, is there a Hamiltonian cycle with total weight at most k?
Transformation t:
Map G, a given instance of the HCP to a weighted complete graph G’,
an instance of the TSP by assigning 1 as the edge weight to each
edge in G and adding an edge of weight 2 between any pair of
nonadjacent vertices in G.
Let k be n, the total vertex number in G.

19

Formal Definition of NP-Completeness
A decision problem D is said to be NP-complete if
1. It belongs to class NP.
2. Every problem in NP is polynomially reducible to D.
The class of NP-complete problems is called NPC. NP
Cook’s theorem (1971): discover the first NP-complete NP-complete
problem, CNF-satisfiability problem.
Show a decision problem is NP-complete
Show that the problem is NP P
Show that a known NP-complete problem can be
transformed to the problem in question in polynomial
time. (transitivity of polynomial reduction)
Practical value of NP-completeness

20

Algorithm chapter 10

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Algorithm chapter 10