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Algorithm chapter 8

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  • 1. Dynamic Programming
  • 2. Dynamic Programming Dynamic Programming is a general algorithm design technique.“Programming” here means “planning”. Invented by American mathematician Richard Bellman in the 1950s tosolve optimization problems Main idea: solve several smaller (overlapping) subproblems record solutions in a table so that each subproblem is only solved once final state of the table will be (or contain) solution Dynamic programming vs. divide-and-conquer partition a problem into overlapping subproblems and independent ones store and not store solutions to subproblems
  • 3. Example: Fibonacci numbers• Recall definition of Fibonacci numbers: f(0) = 0 f(1) = 1 f(n) = f(n-1) + f(n-2)• Computing the nth Fibonacci number recursively (top-down): f(n) f(n-1) + f(n-2)f(n-2) + f(n-3) f(n-3) + f(n-4) ...
  • 4. Example: Fibonacci numbers Computing the nth fibonacci number using bottom-up iteration:• f(0) = 0• f(1) = 1• f(2) = 0+1 = 1• f(3) = 1+1 = 2• f(4) = 1+2 = 3 ALGORITHM Fib(n)• f(5) = 2+3 = 5 F[0] 0, F[1] 1• for i 2 to n do• F[i] F[i-1] + F[i-2]• return F[n]• f(n-2) =• f(n-1) = extra space• f(n) = f(n-1) + f(n-2)
  • 5. Examples of Dynamic Programming Algorithms Computing binomial coefficients Warshall’s algorithm for transitive closure Floyd’s algorithms for all-pairs shortest paths Constructing an optimal binary search tree Some instances of difficult discrete optimizationproblems: knapsack
  • 6. Computing Binomial Coefficients A binomial coefficient, denoted C(n, k), is the number of combinations of k elements from an n-element set (0 ≤ k ≤ n). Recurrence relation (a problem 2 overlapping problems) C(n, k) = C(n-1, k-1) + C(n-1, k), for n > k > 0, and C(n, 0) = C(n, n) = 1 Dynamic programming solution: Record the values of the binomial coefficients in a table of n+1 rows and k+1 columns, numbered from 0 to n and 0 to k respectively. 1 1 1 1 2 1 1 3 3 1 1 4 6 4 1 1 5 10 10 5 1 …
  • 7. Transitive ClosureThe transitive closure of a directed graph with n vertices can be defined as the nxn matrix T = {tij}, in which the element in the ith row (1 ≤ i ≤ n) and the jth column (1 ≤ j ≤ n) is 1 if there exists a nontrivial directed path (i.e., a directed path of a positive length) from the ith vertex to the jth vertex; otherwise, tij is 0.Graph traversal-based algorithm and Warshall’s algorithm 3 3 1 1 4 4 0 0 1 0 2 0 0 1 0 2 1 0 0 1 1 1 1 1 0 0 0 0 0 0 0 0 Adjacency matrix 1 1 1 1 0 1 0 0 Transitive closure
  • 8. Warshall’s Algorithm• Main idea: Use a bottom-up method to construct the transitive closure of a givendigraph with n vertices through a series of nxn boolean matrices: R(0),…, R(k-1), R(k) , …, R(n)The question is: how to obtain R(k) from R(k-1) ? R(k) : rij(k) = 1 in R(k) , iff there is an edge from i to j; or there is a path from i to j going through vertex 1; or there is a path from i to j going through vertex 1 and/or 2; or ... there is a path from i to j going through 1, 2, … and/or k 3 3 3 3 3 1 1 1 1 1 4 4 4 4 2 4 2 2 2 2 R(0) R(1) R(2) R(3) R(4) 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 1 0 0 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 1 0 0 1 1 1 1 1 1 1 1 1 1 1 1Does not allow an Allow 1 to be an Allow 1,2 to be an Allow 1,2,3 to be an Allow 1,2,3,4 to be anintermediate node intermediate node intermediate node intermediate node intermediate node
  • 9. Warshall’s AlgorithmIn the kth stage: to determine R(k) is to determine if a path existsbetween two vertices i, j using just vertices among 1,…,k {rij(k) = 1: rij(k-1) = 1 or (path using just 1 ,…,k-1) (rik(k-1) = 1 and rkj(k-1)) = 1 (path from i to k and from k to i using just 1 ,…,k-1) k kth stagei Rule to determine whether rij (k) should be 1 in R(k) : • If an element rij is 1 in R(k-1) , it remains 1 in R(k) . j • If an element rij is 0 in R(k-1) ,it has to be changed to 1 in R(k) iff the element in its row i and column k and the element in its column j and row k are both 1’s in R(k-1).
  • 10. Floyd’s Algorithm: All pairs shortest paths All pairs shortest paths problem: In a weighted graph, dij(k) = length of thefind shortest paths between every pair of vertices. shortest path from i to j with each vertexApplicable to: undirected and directed weighted graphs; numbered no higherno negative weight. than k.Same idea as the Warshall’s algorithm : constructsolution through series of matrices D(0) , D(1), …, D(n)Example: 4 3 0 ∞ 4 ∞ 0 ∞ 4 ∞ 1 1 0 6 3 1 0 5 3 1 ∞ ∞ 0 ∞ 6 ∞ ∞ 0 ∞ 1 5 ∞ 5 1 0 6 5 1 0 4 2 3 weight matrix distance matrix
  • 11. Floyd’s AlgorithmD(k) : allow 1, 2, …, k to be intermediate vertices.In the kth stage, determine whether the introduction of k as a new eligibleintermediate vertex will bring about a shorter path from i to j.dij(k) = min{dij(k-1) , dik(k-1) + dkj(k-1} for k ≥ 1, dij(0) = wij dik(k-1) k i dkj(k-1) kth stage dij(k-1) j
  • 12. General CommentsThe crucial step in designing a dynamicprogramming algorithm: Deriving a recurrence relating a solution to the problem’s current instance with solutions of its smaller ( and overlapping) subinstances.
  • 13. The Knapsack Problem and Memory Functions (1)The problem Find the most valuable subset of the given n items that fit into a knapsack of capacity W.Consider the following subproblem P(i, j) Find the most valuable subset of the first i items that fit into a knapsack of capacity j, where 1 ≤ i ≤ n, and 1≤ j ≤ W Let V[i, j] be the value of an optimal solution to the above subproblem P(i, j). Goal: V[n, W] The question: What is the recurrence relation that expresses a solution to this instance in terms of solutions to smaller subinstances?
  • 14. The Knapsack Problem and Memory Functions (2)The Recurrence Two possibilities for the most valuable subset for the subproblem P(i, j) It does not include the ith item: V[i, j] = V[i-1, j] It includes the ith item: V[i, j] = vi+ V[i-1, j – wi]V[i, j] = max{V[i-1, j], vi+ V[i-1, j – wi] }, if j – wi ≥ 0 { V[i-1, j] if j – wi < 0V[0, j] = 0 for j ≥ 0 and V[i, 0] = 0 for i ≥ 0
  • 15. Memory Functions Memory functions: a combination of the top-down and bottom- up method. The idea is to solve the subproblems that are necessary and do it only once. Top-down: solve common subproblems more than once. Bottom-up: Solve subproblems whose solution are not necessary for the solving the original problem.ALGORITHM MFKnapsack(i, j)if V[i, j] < 0 //if subproblem P(i, j) hasn’t been solved yet. if j < Weights[i] value MFKnapsack(i – 1, j) else value max(MFKnapsack(i – 1, j), values[I] + MFKnapsck( i – 1, j – Weights[i])) V[i, j] valuereturn V[i, j]