CPSC 411 Design and Analysis of Algorithms Set 4: Greedy Algorithms Prof. Jennifer Welch Spring 2011...
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Transcript of CPSC 411 Design and Analysis of Algorithms Set 4: Greedy Algorithms Prof. Jennifer Welch Spring 2011...
CPSC 411Design and Analysis
of Algorithms
Set 4: Greedy AlgorithmsProf. Jennifer Welch
Spring 2011
CPSC 411, Spring 2011: Set 4 1
CPSC 411, Spring 2011: Set 4 2
Greedy Algorithm Paradigm Characteristics of greedy algorithms:
make a sequence of choices each choice is the one that seems best so far,
only depends on what's been done so far choice produces a smaller problem to be
solved In order for greedy heuristic to solve the
problem, it must be that the optimal solution to the big problem contains optimal solutions to subproblems
CPSC 411, Spring 2011: Set 4 3
Designing a Greedy Algorithm Cast the problem so that we make a greedy
(locally optimal) choice and are left with one subproblem
Prove there is always a (globally) optimal solution to the original problem that makes the greedy choice
Show that the choice together with an optimal solution to the subproblem gives an optimal solution to the original problem
CPSC 411, Spring 2011: Set 4 4
Some Greedy Algorithms fractional knapsack algorithm Huffman codes Kruskal's MST algorithm Prim's MST algorithm Dijkstra's SSSP algorithm …
CPSC 411, Spring 2011: Set 4 5
Knapsack Problem There are n different items in a store Item i :
weighs wi pounds worth $vi
A thief breaks in Can carry up to W pounds in his
knapsack What should he take to maximize the
value of his haul?
CPSC 411, Spring 2011: Set 4 6
0-1 vs. Fractional Knapsack 0-1 Knapsack Problem:
the items cannot be divided thief must take entire item or leave it
behind Fractional Knapsack Problem:
thief can take partial items for instance, items are liquids or powders solvable with a greedy algorithm…
CPSC 411, Spring 2011: Set 4 7
Greedy Fractional Knapsack Algorithm Sort items in decreasing order of
value per pound While still room in the knapsack (limit
of W pounds) do consider next item in sorted list take as much as possible (all there is or as
much as will fit) O(n log n) running time (for the sort)
CPSC 411, Spring 2011: Set 4 8
Greedy 0-1 Knapsack Alg? 3 items:
item 1 weighs 10 lbs, worth $60 ($6/lb) item 2 weighs 20 lbs, worth $100 ($5/lb) item 3 weighs 30 lbs, worth $120 ($4/lb)
knapsack can hold 50 lbs greedy strategy:
take item 1 take item 2 no room for item 3
CPSC 411, Spring 2011: Set 4 9
0-1 Knapsack Problem Taking item 1 is a big mistake globally
although looks good locally Later we'll see a different algorithm
design paradigm that does work for this problem
CPSC 411, Spring 2011: Set 4 10
Finding Optimal Code Input:
data file of characters and number of occurrences of each character
Output: a binary encoding of each character so
that the data file can be represented as efficiently as possible
"optimal code"
CPSC 411, Spring 2011: Set 4 11
Huffman Code Idea: use short codes for more
frequent characters and long codes for less frequent
char a b c d e f total bits
# 45 13 12 16 9 5
fixed 000 001 010 011 100 101 300
variable
0 101 100 111 1101 1100 224
CPSC 411, Spring 2011: Set 4 12
How to Decode? With fixed length code, easy:
break up into 3's, for instance For variable length code, ensure that
no character's code is the prefix of another no ambiguity
101111110100b d e a a
CPSC 411, Spring 2011: Set 4 13
Binary Tree Representation
0 1
0 1 0
0 1 0 1 0 1
a b c d e f
fixed length code
cost of code is sum,over all chars c, ofnumber of occurrencesof c times depth of c inthe tree
CPSC 411, Spring 2011: Set 4 14
0 1
f e
Binary Tree Representation10
0 1
0 1 0 1
a
bc d
variable length code
cost of code is sum,over all chars c, ofnumber of occurrencesof c times depth of c inthe tree
CPSC 411, Spring 2011: Set 4 15
Algorithm to Construct Tree Representing Huffman Code Given set C of n chars, c occurs f[c] times insert each c into priority queue Q using f[c]
as key for i := 1 to n-1 do
x := extract-min(Q) y := extract-min(Q) make a new node z w/ left child x (label edge 0),
right child y (label edge 1), and f[z] = f[x] + f[y] insert z into Q
CPSC 411, Spring 2011: Set 4 16
<board work>
Graphs Directed graph G = (V,E)
V is a finite set of vertices E is the edge set, a binary relation on E (ordered pairs of
vertice) In an undirected graph, edges are unordered pairs of
vertices If (u,v) is in E, then v is adjacent to u and (u,v) is
incident from u and incident to v; v is neighbor of u in an undirected graph, u is also adjacent to v, and (u,v) is
incident on u and v Degree of a vertex is number of edges incident from
or to (or on) the vertex
CPSC 411, Spring 2011: Set 4 17
More on Graphs A path of length k from vertex u to vertex u’ is
a sequence of k+1 vertices s.t. there is an edge from each vertex to the next in the sequence In this case, u’ is reachable from u
A path is simple if no vertices are repeated A path forms a cycle if last vertex = first vertex A cycle is simple if no vertices are repeated
other than first and last
CPSC 411, Spring 2011: Set 4 18
Graph Representations So far, we have discussed graphs purely as
mathematical concepts How can we represent a graph in a
computer program? We need some data structures.
Two most common representations are: adjacency list – best for sparse graphs (few edges) adjacency matrix – best for dense graphs (lots of
edges)
CPSC 411, Spring 2011: Set 4 19
Adjacency List Representation
Given graph G = (V,E) array Adj of |V| linked lists, one for each vertex in V The adjacency list for vertex u, Adj[u], contains all
vertices v s.t. (u,v) is in E each vertex in the list might be just a string,
representing its name, or it might be some other data structure representing the vertex, or it might be a pointer to the data structure for that vertex
Requires Θ(V+E) space (memory) Requires Θ(degree(u)) time to check if (u,v) is in E
CPSC 411, Spring 2011: Set 4 20
Adjacency Matrix Representation
Given graph G = (V,E) Number the vertices 1, 2, ..., |V| Use a |V| x |V| matrix A with (i,j)-entry of A
being 1 if (i,j) is in E 0 if (i,j) is not in E
Requires Θ(V2) space (memory) Requires Θ(1) time to check if (u,v) is in E
CPSC 411, Spring 2011: Set 4 21
More on Graph Representations Read Ch 22, Sec 1 for
more about directed vs. undirected graphs how to represented weighted graphs
(some value is associated with each edge) pros and cons of adjacency list vs.
adjacency matrix
CPSC 411, Spring 2011: Set 4 22
CPSC 411, Spring 2011: Set 4 23
Minimum Spanning Tree
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Given a connected undirected graph with edge weights,find subset of edges that spans all the nodes,creates no cycle, and minimizes sum of weights
CPSC 411, Spring 2011: Set 4 24
Facts About MSTs There can be many spanning trees of
a graph In fact, there can be many minimum
spanning trees of a graph But if every edge has a unique
weight, then there is a unique MST
CPSC 411, Spring 2011: Set 4 25
Uniqueness of MST Suppose in contradiction there are 2
MSTs, M1 and M2. Let e be edge with minimum weight that
is in one MST but not the other (say it is in M1).
If e is added to M2, a cycle is formed. Let e' be an edge in the cycle that is not
in M1
CPSC 411, Spring 2011: Set 4 26
Uniqueness of MST
e: in M1 but not M2
e': in M2 but not M1; wt of e' is less than wt of e,by choice of e
M2:
Replacing e with e' creates a new MST M3 whose weight is less than that of M2
CPSC 411, Spring 2011: Set 4 27
Kruskal's MST algorithm
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consider the edges in increasing order of weight,add in an edge iff it does not cause a cycle
CPSC 411, Spring 2011: Set 4 28
Why is Kruskal's Greedy? Algorithm manages a set of edges s.t.
these edges are a subset of some MST At each iteration:
choose an edge so that the MST-subset property remains true
subproblem left is to do the same with the remaining edges
Always try to add cheapest available edge that will not violate the tree property locally optimal choice
CPSC 411, Spring 2011: Set 4 29
Correctness of Kruskal's Alg. Let e1, e2, …, en-1 be sequence of
edges chosen Clearly they form a spanning tree Suppose it is not minimum weight Let ei be the edge where the
algorithm goes wrong {e1,…,ei-1} is part of some MST M but {e1,…,ei} is not part of any MST
CPSC 411, Spring 2011: Set 4 30
Correctness of Kruskal's Alg.
gray edges are part of MST M, which contains e1 to ei-1,but not ei
M: ei, forms a cycle in M
e* :min wt. edge in cycle not ine1 to ei-1
replacing e* w/ ei formsa spanning tree withsmaller weight than M,contradiction!
wt(e*) > wt(ei)
CPSC 411, Spring 2011: Set 4 31
Note on Correctness Proof Argument on previous slide works for
case when every edge has a unique weight
Algorithm also works when edge weights are not necessarily unique
Modify proof on previous slide: contradiction is reached to assumption that ei is not part of any MST
CPSC 411, Spring 2011: Set 4 32
Implementing Kruskal's Alg. Sort edges by weight
efficient algorithms known How to test quickly if adding in the
next edge would cause a cycle? use disjoint set data structure, later
CPSC 411, Spring 2011: Set 4 33
Another Greedy MST Alg. Kruskal's algorithm maintains a forest
that grows until it forms a spanning tree Alternative idea is keep just one tree and
grow it until it spans all the nodes Prim's algorithm
At each iteration, choose the minimum weight outgoing edge to add greedy!