15-451 Algorithms 10/26/06
* network flow
* bipartite matching
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Today and next time we are going to talk about an important
algorithmic problem called the Network Flow problem. It's important
because it can be used to express and solve a lot of different things.
In O.R. they have entire courses devoted to the topic.
NETWORK FLOW
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- directed graph. Source node. Sink node. Each (directed) edge has a
capacity. (Let's say these are integers.) Goal is to flow as much
stuff as possible from source to sink [not just one path]. E.g.,
s-->a, cap = 4. a-->c, cap=3. c-->t, cap = 2.
s-->b, cap = 2. b-->d, cap=3. d-->t, cap = 4.
c-->b, cap = 1. b-->c, cap=2.
E.g., want to route message traffic from source to sink, and the
capacities tell us how much bandwidth we're allowed on each edge.
Formally, rules are:
Capacity constraint: on any edge, f(u,v) <= c(u,v)
Flow conservation: for any vertex except s and t, flow in = flow out.
What is the max flow here? Answer: 5.
How can you see that above max flow was really maximum? Notice, this
flow saturates the a-->c and s-->b edges. And, if you remove these,
you disconnect t from s. In other words, the graph has a "s-t cut" of
size 5. (a set of edges of total capacity 5 that if you remove
disconnects the source from the sink). So, the point is that any
unit of stuff going from s to t must take up at least 1 unit of
capacity in these pipes. So, we know we're optimal.
We just argued that in general, max flow can't be larger than any s-t cut.
DEFINITION: an s-t cut is a set of edges whose removal disconnects t from s.
Or, formally, a cut is a partition of vertex set into A and B where s
is in A and t is in B. (The edges of the cut are then all edges going
from A to B).
DEFINITION: the capacity of cut (A,B) is the sum of capacities of edges in
the cut. Or, in the formal viewpoint, it is the sum of capacities of
all edges going from A to B. (Don't include edges from B to A.)
Easy fact we showed above: max s-t flow <= min s-t cut.
How to find a maximum flow and prove it's correct? Here's a very
natural strategy: find a path from s to t and push as much flow on it
as possible. Then look at the leftover graph and repeat until there
is no longer any path with capacity left to push any more flow on.
Of course, we need to *prove* that this works: we can't somehow end up
at a suboptimal solution by making bad choices along the way.
This is called the Ford-Fulkerson algorithm.
Basic Ford-Fulkerson algorithm
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While there exists an s-->t path P of positive residual (leftover) capacity:
push maximum possible flow along P (saturating at least one edge on it)
[These are called "augmenting paths"]
Definition: "RESIDUAL CAPACITY" c_f(u,v) = c(u,v) - f(u,v)
where we make the convention that f(v,u) = -f(u,v).
Definition: the RESIDUAL GRAPH is the directed graph with all edges of
positive residual capacity, each one labeled by its residual
capacity. Note: may include "back-edges" of original graph.
--> Example on above graph: start with path s-->b-->c-->t. Draw flow using
"[flow/capacity]" notation, and draw residual graph.
Note: residual capacity can be *larger* than original capacity if we
had flow going in opposite direction. E.g., residual capacity on the
c-->b edge is now 3.
CLAIM: Any legal flow in the Residual Graph can be added to the flow
so far and get a legal flow in the original graph.
PROOF: (1) flow-in = flow-out condition is always satisfied when you
add two flows. (2) Capacity constraints satisfied by defn of residual.
Why is the residual graph good to have? One reason: now left with
same type of problem we started with. Can use any black-box
path-finding method (e.g., DFS).
--> Note: running time is not necessarily so great - it depends on the
choices we make. E.g., standard bad-choice graph. After we prove
correctness, we'll worry about about algorithms to fix this problem
(next time).
Algorithm by design finds a legal flow. Why is it maximum?
Proof: Let's look at the final residual graph. This graph must have s
and t disconnected by definition of the algorithm. Let A be
component containing s and B = rest. Now, let C = capacity of the (A,B)
cut in the *original* graph --- so we know we can't do better than C.
The claim is that we in fact *did* find a flow of value C (which
therefore implies it is maximum). Here's why: let's look at what
happens to the residual capacity of the cut after each iteration of
the algorithm. Say in some iteration we found a path with k units of
flow. Then, even if the path zig-zagged between A and B, every time
we went A-->B we added k to flow from A to B (and subtracted k from
the residual capacity) and every time we went B-->A we took away k
from this flow (and added k to the residual capacity). And, we went
from A-->B exactly one more time than we went from B-->A. So, the
residual capacity went down by exactly k. So, the drop in capacity is
equal to the increase in flow. Since at the end the residual capacity
is zero (remember how we defined A and B) this means the total flow is
equal to C.
So, we've found a flow of value EQUAL to the capacity of this cut. We
know we can't do better, so this must be a max flow, and C must be a
minimum cut.
Note, we've actually proven the nonobvious MAXFLOW-MINCUT theorem:
In any graph, max flow from s to t = capacity of minimum (s,t)-cut.
We started with saying max flow <= min cut. But now we've argued that
this algorithm finds a flow of value equal to SOME cut, so it can't be
less than the MINIMUM.
We have also proven the INTEGRAL-FLOW THEOREM: if all capacities are
integers, then there is a max flow in which all flows are integers.
[This seems obvious, but you'll use it to show something that's not at
all obvious on hwk 5.]
Bipartite Maximum matching problem
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Say we wanted to be more sophisticated about assigning homework
presentation slots to groups. We could ask each group to list the
slots acceptable to them, and then draw this as a bipartite graph:
group slot
----- ----
A 1
B 2
C 3
Edge if acceptable: e.g, (A 1) (A 2) (B 1) (B 3) (C 2) (C 3)
This is a BIPARTITE GRAPH: a graph with two sides L and R, and all
edges go between L and R.
A MATCHING is a set of edges with no endpoints in common. What we
want here is a PERFECT MATCHING - a matching that connects every point
on the left hand side with some point on the right hand side. What's
a perfect matching here? More generally (say there is no perfect
matching) we want a MAXIMUM MATCHING - a matching with maximum
possible number of edges.
Algorithm to solve:
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1. Set up fake "start" node S connected to all in L. Connect all in R to
a fake "end" node T. Orient all edges left-to-right and give each a
capacity of 1.
2. Find a max flow from S to T using Ford-Fulkerson.
3. Output matching corresponding to flow.
This finds a legal matching because edges from R to T have capacity 1,
so the matching can't have two edges into same node, and similarly the
edges from S to L have cap=1, so you can't have two edges leaving same
node in L. It's maximum because any matching gives you a flow of the
same value. (So if there was a better matching, we wouldn't be at a
maximum flow).
What about the number of iterations of path-finding? This is at most
the #edges in the matching since each augmenting path gives us 1 new edge.
[Run alg on above example]. Notice a neat fact: say you start
matching A->1, C->3. These are bad choices. But, the augmenting
path automatically undoes them as it improves the flow!
Matchings come up as subroutines in a lot of problems like matching
up suppliers to customers, or cell-phones to cell-stations when you
have overlapping cells. Also as a basic subroutine in other
algorithmic problems.