Minimum Spanning Trees A tree (i.e., connected, acyclic graph) which contains all the vertices of the graph Minimum Spanning Tree Spanning tree with the minimum sum of weights a b c d e h g f i 4 8 7 11 1 2 14 9 10 6

Prim’s Algorithm Starts from an arbitrary “root”: VA = {a} At each step: Find a light edge crossing (VA, V - VA) Add this edge to set A (The edges in set A always form a single tree) Repeat until the tree spans all vertices 8 7 b c d 4 9 2 a 11 i 14 e 4 7 6 8 10 h g f 1 2

Example 0         Q = {a, b, c, d, e, f, g, h, i} VA =  4 8 7 11 1 2 14 9 10 6 0         Q = {a, b, c, d, e, f, g, h, i} VA =  Extract-MIN(Q)  a 4    key [b] = 4  [b] = a key [h] = 8  [h] = a 4      8  Q = {b, c, d, e, f, g, h, i} VA = {a} Extract-MIN(Q)  b 8 7 b c d 4 9  2  a 11 i 14 e 4 7 6 8 10 h g f 1 2    8

key [h] = 8  [h] = a - unchanged 8     8  Extract-MIN(Q)  c Q = {c, d, e, f, g, h, i} VA = {a, b} key [c] = 8  [c] = b key [h] = 8  [h] = a - unchanged 8     8  Extract-MIN(Q)  c 4  8  8 a b c d e h g f i 4 8 7 11 1 2 14 9 10 6  Q = {d, e, f, g, h, i} VA = {a, b, c} key [d] = 7  [d] = c key [f] = 4  [f] = c key [i] = 2  [i] = c 7  4  8 2 Extract-MIN(Q)  i 4  8 7 a b c d e h g f i 4 8 7 11 1 2 14 9 10 6 2 4

key [d] = 7  [d] = c unchanged key [e] = 10  [e] = f 7 10 2 7 4 7  8 2 Q = {d, e, f, g, h} VA = {a, b, c, i} key [h] = 7  [h] = i key [g] = 6  [g] = i 7  4 6 7 Extract-MIN(Q)  f a b c d e h g f i 4 8 7 11 1 2 14 9 10 6 7 6 4 7  6 8 2 Q = {d, e, g, h} VA = {a, b, c, i, f} key [g] = 2  [g] = f key [d] = 7  [d] = c unchanged key [e] = 10  [e] = f 7 10 2 7 Extract-MIN(Q)  g a b c d e h g f i 4 8 7 11 1 2 14 9 10 6 10 2

Q = {d, e, h} VA = {a, b, c, i, f, g} key [h] = 1  [h] = g 7 10 1 4 7 10 2 8 a b c d e h g f i 4 8 7 11 1 2 14 9 10 6 Q = {d, e, h} VA = {a, b, c, i, f, g} key [h] = 1  [h] = g 7 10 1 Extract-MIN(Q)  h 1 4 7 10 1 2 8 a b c d e h g f i 4 8 7 11 1 2 14 9 10 6 Q = {d, e} VA = {a, b, c, i, f, g, h} 7 10 Extract-MIN(Q)  d

Q = {e} VA = {a, b, c, i, f, g, h, d} key [e] = 9  [e] = d 9 Extract-MIN(Q)  e Q =  VA = {a, b, c, i, f, g, h, d, e} 4 7 10 1 2 8 a b c d e h g f i 4 8 7 11 1 2 14 9 10 6 9

PRIM(V, E, w, r) % r : starting vertex Q ←  for each u  V do key[u] ← ∞ π[u] ← NIL INSERT(Q, u) DECREASE-KEY(Q, r, 0) % key[r] ← 0 while Q   do u ← EXTRACT-MIN(Q) for each v  Adj[u] do if v  Q and w(u, v) < key[v] then π[v] ← u DECREASE-KEY(Q, v, w(u, v)) Total time: O(VlgV + ElgV) = O(ElgV) O(V) if Q is implemented as a min-heap O(lgV) Min-heap operations: O(VlgV) Executed |V| times Takes O(lgV) Executed O(E) times total Constant Takes O(lgV) O(ElgV)

Prim’s Algorithm Total time: O(ElgV ) Prim’s algorithm is a “greedy” algorithm Greedy algorithms find solutions based on a sequence of choices which are “locally” optimal at each step. Nevertheless, Prim’s greedy strategy produces a globally optimum solution!

Kruskal’s Algorithm Start with each vertex being its own component Repeatedly merge two components into one by choosing the lightest edge that connects them Which components to consider at each iteration? Scan the set of edges in monotonically increasing order by weight. Choose the smallest edge. a b c d e h g f i 4 8 7 11 1 2 14 9 10 6 We would add edge (c, f)

Example a b c d e h g f i 1: (h, g) 2: (c, i), (g, f) Add (h, g) Add (c, i) Add (g, f) Add (a, b) Add (c, f) Ignore (i, g) Add (c, d) Ignore (i, h) Add (a, h) Ignore (b, c) Add (d, e) Ignore (e, f) Ignore (b, h) Ignore (d, f) {g, h}, {a}, {b}, {c},{d},{e},{f},{i} {g, h}, {c, i}, {a}, {b}, {d}, {e}, {f} {g, h, f}, {c, i}, {a}, {b}, {d}, {e} {g, h, f}, {c, i}, {a, b}, {d}, {e} {g, h, f, c, i}, {a, b}, {d}, {e} {g, h, f, c, i, d}, {a, b}, {e} {g, h, f, c, i, d, a, b}, {e} {g, h, f, c, i, d, a, b, e} a b c d e h g f i 4 8 7 11 1 2 14 9 10 6 1: (h, g) 2: (c, i), (g, f) 4: (a, b), (c, f) 6: (i, g) 7: (c, d), (i, h) 8: (a, h), (b, c) 9: (d, e) 10: (e, f) 11: (b, h) 14: (d, f) {a}, {b}, {c}, {d}, {e}, {f}, {g}, {h}, {i}

Operations on Disjoint Data Sets Kruskal’s Alg. uses Disjoint Data Sets (UNION-FIND : Chapter 21) to determine whether an edge connects vertices in different components MAKE-SET(u) – creates a new set whose only member is u FIND-SET(u) – returns a representative element from the set that contains u. It returns the same value for any element in the set UNION(u, v) – unites the sets that contain u and v, say Su and Sv E.g.: Su = {r, s, t, u}, Sv = {v, x, y} UNION (u, v) = {r, s, t, u, v, x, y} We had seen earlier that FIND-SET can be done in O(lgn) or O(1) time and UNION operation can be done in O(1) (see Chapter 21)

KRUSKAL(V, E, w) Running time: O(V+ElgE+ElgV)  O(ElgE) O(V) O(ElgE) for each vertex v  V do MAKE-SET(v) sort E into non-decreasing order by w for each (u, v) taken from the sorted list do if FIND-SET(u)  FIND-SET(v) then A ← A  {(u, v)} UNION(u, v) return A Running time: O(V+ElgE+ElgV)  O(ElgE) Implemented by using the disjoint-set data structure (UNION-FIND) Kruskal’s algorithm is “greedy” It produces a globally optimum solution O(V) O(ElgE) O(E) O(lgV)

Another Example for Prim’s Method a b c d e f D[.] = 2 0 10 1   1 f a 2 3 7 S 10 b c 2 a b c d e f a 0 2 7   1 b 2 0 10 1   c 7 10 0 8 2  d  1 8 0 9  e   2 9 0 3 f 1    3 0 e 8 1 9 d new D[i] = Min{ D[i], w(k, i) } where k is the newly-selected node and w[.] is the distance between k and i

new D[i] = Min{ D[i], w(k, i) } a b c d e f L [.] = 2 0 10 1   1 f a 2 3 a b c d e f new L [.] = 2 0 8 1 9  7 10 b c 2 a b c d e f a 0 2 7   1 b 2 0 10 1   c 7 10 0 8 2  d  1 8 0 9  e   2 9 0 3 f 1    3 0 e 8 1 9 d new D[i] = Min{ D[i], w(k, i) } where k is the newly-selected node and w[.] is the distance between k and i

new D[i] = Min{ D[i], w(k, i) } a b c d e f L [.] = 2 0 8 1 9  1 f a 2 3 a b c d e f new L [.] = 2 0 7 1 9 1 7 10 b c 2 a b c d e f a 0 2 7   1 b 2 0 10 1   c 7 10 0 8 2  d  1 8 0 9  e   2 9 0 3 f 1    3 0 e 8 1 9 d new D[i] = Min{ D[i], w(k, i) } where k is the newly-selected node and w[.] is the distance between k and i

f a b c e d a b c d e f L [.] = 2 0 7 1 9 1 a b c d e f 3 a b c d e f new L [.] = 2 0 7 1 3 1 10 b c 2 a b c d e f a 0 2 7   1 b 2 0 10 1   c 7 10 0 8 2  d  1 8 0 9  e   2 9 0 3 f 1    3 0 e 8 1 9 d

f a b c e d a b c d e f L [.] = 2 0 7 1 3 1 a b c d e f new L [.] = 2 0 2 1 3 1 10 b c 2 a b c d e f a 0 2 7   1 b 2 0 10 1   c 7 10 0 8 2  d  1 8 0 9  e   2 9 0 3 f 1    3 0 e 8 1 9 d

Running time: (V2) (array representation) a b c d e f L [.] = 2 0 2 1 3 1 1 f a a b c d e f new L [.] = 2 0 2 1 3 1 2 7 3 10 b c 2 a b c d e f a 0 2 7   1 b 2 0 10 1   c 7 10 0 8 2  d  1 8 0 9  e   2 9 0 3 f 1    3 0 e 8 1 9 d Running time: (V2) (array representation) (ElgV) (Min-Heap+Adjacency List) Which one is better?

Greedy MST Methods Prim’s method is fastest. O(n2) (worst case) O(E log n) if a Min Heap is used to keep track of distances of vertices to partially built tree. If e=O(n2), MinHeap is not a good idea! Kruskal’s uses union-find trees to run in O(E log n) time.

Parallel MST Algorithm (Prim’s) P processors, n=|V| vertices Each processor is assigned n/p vertices (Pi gets the set Vi) Each PE holds the n/p columns of A and n/p elements of d[] array P0 P1 Pi Pp-1 d[.] . . . . . . n/p columns | | | | | | | | | | | | ….. ….. A

Parallel MST Algorithm (Prim’s) 1. Initialize: Vt := {r}; d[k] =  for all k (except d[r] = 0) 2. P0 broadcasts selectedV = r using one-to-all broadcast. 3. The PE responsible for "selectedV" marks it as belonging to set Vt. 4. For v = 2 to n=|V| do 5. Each Pi updates d[k] = Min[d[k], w(selectedV, k)] for all k  Vi 6. Each Pi computes MIN-di =(minimum d[] value among its unselected elements) 7. PEs perform a "global minimum" using MIN-di values and store the result in P0. Call the winning vertex, selectedV. 8. P0 broadcasts "selectedV" using one-to-all broadcast. 9. The PE responsible for "selectedV" marks it as belonging to set Vt. 10. EndFor

Parallel MST Algorithm (Prim’s) TIME COMPLEXITY ANALYSIS: E=O(n2) then Tseq = n2 (Hypercube) Tpar = n*(n/p) + n*logp computation + communication (Mesh) Tpar = n*(n/p) + n * Sqrt(p) The algorithm is cost-optimal on a hypercube if plogp/n =O(1)

Dijkstra’s SSSP Algorithm (adjacency matrix) a b c d e f L [.] = 2 0 10 1   1 f a 2 3 7 S 10 b c 2 a b c d e f a 0 2 7   1 b 2 0 10 1   c 7 10 0 8 2  d  1 8 0 9  e   2 9 0 3 f 1    3 0 e 8 1 9 d new L[i] = Min{ L[i], L[k] + W[k, i] } where k is the newly-selected intermediate node and W[.] is the distance between k and i

SSSP cont. f a b c e d new L[i] = Min{ L[i], L[k] + W[k, i] } a b c d e f L [.] = 2 0 10 1   1 f a 2 3 a b c d e f new L [.] = 2 0 9 1 10  7 10 b c 2 a b c d e f a 0 2 7   1 b 2 0 10 1   c 7 10 0 8 2  d  1 8 0 9  e   2 9 0 3 f 1    3 0 e 8 1 9 d new L[i] = Min{ L[i], L[k] + W[k, i] } where k is the newly-selected intermediate node and W[.] is the distance between k and i

new L[i] = Min{ L[i], L[k] + W[k, i] } a b c d e f L [.] = 2 0 9 1 10  1 f a 2 3 a b c d e f new L [.] = 2 0 9 1 10 3 7 10 b c 2 a b c d e f a 0 2 7   1 b 2 0 10 1   c 7 10 0 8 2  d  1 8 0 9  e   2 9 0 3 f 1    3 0 e 8 1 9 d new L[i] = Min{ L[i], L[k] + W[k, i] } where k is the newly-selected intermediate node and W[.] is the distance between k and i

f a b c e d a b c d e f L [.] = 2 0 9 1 10 3 a b c d e f 7 3 a b c d e f new L [.] = 2 0 9 1 6 3 10 b c 2 a b c d e f a 0 2 7   1 b 2 0 10 1   c 7 10 0 8 2  d  1 8 0 9  e   2 9 0 3 f 1    3 0 e 8 1 9 d

f a b c e d a b c d e f L [.] = 2 0 9 1 6 3 a b c d e f 7 3 a b c d e f new L [.] = 2 0 8 1 6 3 10 b c 2 a b c d e f a 0 2 7   1 b 2 0 10 1   c 7 10 0 8 2  d  1 8 0 9  e   2 9 0 3 f 1    3 0 e 8 1 9 d

Running time: (V2) (array representation) a b c d e f L [.] = 2 0 8 1 6 3 1 f a a b c d e f new L [.] = 2 0 8 1 6 3 2 7 3 10 b c 2 a b c d e f a 0 2 7   1 b 2 0 10 1   c 7 10 0 8 2  d  1 8 0 9  e   2 9 0 3 f 1    3 0 e 8 1 9 d Running time: (V2) (array representation) (ElgV) (Min-Heap+Adjacency List) Which one is better?

Task Partitioning for Parallel SSSP Algorithm P processors, n=|V| vertices Each processor is assigned n/p vertices (Pi gets the set Vi) Each PE holds the n/p columns of A and n/p elements of L[] array as shown below P0 P1 Pi Pp-1 L[.] . . . . . . n/p columns | | | | | | | | | | | | ….. ….. A

Parallel SSSP Algorithm (Dijkstra’s) 1. Initialize: Vt := {r}; L[k] =  for all k except L[r] = 0; 2. P0 broadcasts selectedV = r using one-to-all broadcast. 3. The PE responsible for "selectedV" marks it as belonging to set Vt. 4. For v = 2 to n=|V| do 5. Each Pi updates L[k] = Min[ L[k], L(selectedV)+W(selectedV, k) ] for k  Vi 6. Each Pi computes MIN-Li = (minimum L[.] value among its unselected elements) 7. PEs perform a "global minimum" using MIN-Li values and result is stored in P0. Call the winning vertex, selectedV. 8. P0 broadcasts "selectedV" and L[selectedV] using one-to-all broadcast. 9. The PE responsible for "selectedV" marks it as belonging to set Vt. 10. EndFor

Parallel SSSP Algorithm (Dijkstra’s) TIME COMPLEXITY ANALYSIS: In the worst-case, Tseq = n2 (Hypercube) Tpar = n*(n/p) + n*logp computation + communication (Mesh) Tpar = n*(n/p) + n * Sqrt(p) The algorithm is cost-optimal on a hypercube if plogp/n = O(1)