10_Max_Flows_3

10_Max_Flows_3 - 15.082 and 6.855J Max Flows 3 Preflow-Push...

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1 15.082 and 6.855J March 11, 2003 Max Flows 3 Preflow-Push Algorithms

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2 Review of Augmenting Paths At each iteration: maintain a flow x Let G(x) be the residual network At each iteration, find a path from s to t in G(x). In the shortest augmenting path algorithm, we kept distance labels d( ), and we sent flow along the shortest path in G(x).
3 Preflows At each intermediate stages we permit more flow arriving at nodes than leaving (except for s) A preflow is a function x: A R s.t. 0 x u and such that e(i) = j N x ji - j N x ij 0, for all i N – {s, t}. i.e., e(i) = excess at i = net excess flow into node i. The excess is required to be nonnegative.

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4 A Feasible Preflow The excess e(j) at each node j s, t is the flow in minus the flow out. s 3 4 2 5 t 3 3 3 2 2 2 2 1 2 1 0 0 Note: total excess = flow out of s minus flow into t.
5 Active nodes Nodes with positive excess are called active . s 3 4 2 5 t 3 3 3 2 2 2 2 1 2 1 0 0 The preflow push algorithm will try to push flow from active nodes towards the sink, relying on d( ).

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6 Review of Distance Labels Distance labels d( ) are valid for G(x) if i. d(t) = 0 ii. d(i) d(j) + 1 for each (i,j) G(x) Defn. An arc (i,j) is admissible if r ij > 0 and d(i) = d(j) + 1. Lemma. Let d( ) be a valid distance label. Then d(i) is a lower bound on the distance from i to t in the residual network.
7 Push/Relabel, the fundamental subroutine Suppose we have selected an active node i. Procedure Push/Relabel(i) begin if the network contains an admissible arc (i,j) then push δ : = min{ e(i), r ij } units of flow from i to j; else replace d(i) by min{d(j) + 1 : (i,j) A(i) and r ij > 0} end ;

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8 Pushing using current arcs Tail Head Res. Cap Admissible ? 4 1 0 No 4 2 1 No 4 3 4 Yes 4 5 0 No 4 6 2 Yes Suppose that node 4 is active, and has excess. 3 4 1 2 3 5 6 1 3 2 2 2 e(4) = 2 Scan arcs in A(4) one at a time using “Current Arc” till an admissible arc is found. Push on (4,3)
9 Pushing on (4,3) Tail Head Res. Cap Admissible ? 4 1 0 No 4 2 1 No 4 3 4 Yes 4 5 0 No 4 6 2 Yes 3 4 1 2 3 5 6 1 3 2 2 2 e(4) = 2 Push on (4,3) e(3) = 1 Send min (e(4), r 43 ) = 2 units of flow. 2 e(4) = 0 e(3) = 3 For the next push from node 4, start with arc (4,3). Update the residual capacities and excesses.

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10 Goldberg-Tarjan Preflow Push Algorithm Procedure Preprocess begin x :=0; compute the exact distance labels d(i) for each node; x sj := u sj for each arc (s,j) A(s); d(s) := n; end Preflow Push A Algorithm PREFLOW-PUSH; begin preprocess; while there is an active node i do begin select an active node i; push/relabel(i); end; end;
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