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Lectures on
Network Flows
COMP 523: Advanced Algorithmic Techniques
Lecturer: Dariusz Kowalski
Lectures on Network Flows
1
Overview
Previous lectures:
• Dynamic programming
– Weighted interval scheduling
– Sequence alignment
These lectures:
• Network flows
• Applications: largest matching in bipartite graphs
Lectures on Network Flows
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Network
A directed graph G = (V,E) such that
• each directed edge e has its nonnegative
capacity denoted by ce
• there is a node s (source) with no incoming
edges
• there is a node t (target) with no outgoing
u
edges
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u,v - internal
nodes
10
30
s
10
t
20
v
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Flow
s-t flow in G = (V,E) is a function f from E to R+
• capacity condition: for each e, 0  f(e)  ce
• conservation condition: for each internal node v,
∑e in v f(e) = ∑e out v f(e)
• there is a node t (target) with no outgoing edges
Property: ∑e in t f(e) = ∑e out s f(e)
Network:
u
20
u
10
30
s
10
t
20
v
Flow:
20
10
s
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Useful definitions
Given s-t flow f in G = (V,E) and any subset of
nodes S
• f in(S) = ∑e in S f(e)
• f out(S) = ∑e out S f(e)
Property: f in(t) = f out(s)
Example: f in(u,v) = f out(u,v) = 30
Network:
u
20
u
10
30
s
10
t
20
v
Flow: 20
10
s
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10
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20
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Problem(s)
•
•
What is the maximum value of f in(t) (flow) for
a given graph G = (V,E) ?
How to compute it efficiently?
Assumption: capacities are positive integers.
Example: f in(t) = f out(s) = 30
Network:
u
20
u
30
s
Flow: 20
10
10
t
20
v
10
s
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Residual graph
Assume that we are given a flow f in graph G.
Residual graph Gf
• The same nodes, internal and s,t
• For each edge e in E with ce > f(e) we put
weight ce - f(e) (residual capacity)
• For each edge e = (u,v) in E we put weight f(e)
to the backward edge (v,u) (residual capacity)
u
Network: u
20
10
30
s
10
t
20
v
Residual
u
Graph: 20
10
20
t
s
20
10
t
s
0
20
20
10
v
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v
Flow: 20
0
Augmenting path & augmentation
Assume that we are given a flow f in graph G, and
the corresponding residual graph Gf
1. Find a new flow in residual graph - through a path
with no repeating nodes, and value equal to the
minimum capacity on the path (augmenting path)
2. Update residual graph along the path
u
Network: u
20
10
30
s
10
20
v
New 20
10
flow:
1020 10
s
t
10
20
v
Lectures on Network Flows
New
residual
u
graph:
10
20
t
10
20
s
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t
Ford-Fulkerson Algorithm
•
•
Initialize f(e) = 0 for all e
While there is s-t path P in residual graph
–
Augment f through path P and get new f and new residual
graph
Augment f through path P :
• Find minimum capacity on the path
• Go through the path and modify weights
Network: u
20
10
30
s
10
20
v
New
u
u
New
10 residual 20
10
flow: 20
graph:
t
1020 10
10
20
t
s
s
10
10
20
20
v
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Lectures on Network Flows
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Analysis
Correctness:
maximum flow - proof later
termination - each time the flow is an integer and advances by
at least 1 (assumption about integer capacities)
Time: O(mC)
• at most C iterations, where C is the value of the maximum
flow, m is the number of edges
• each iteration takes O(m+n) steps - use DFS to find path P
Network: u
20
10
30
s
10
t
20
v
New
u
10 residual 20
10
graph:
1020 10
10
20
t
s
s
10
10
20
20
v
v
New
flow: 20
u
Lectures on Network Flows
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t
Reminder: Depth-First Search (DFS)
Given a directed graph G = (V,E) of diameter D and the root node
Goal: find a directed rooted spanning tree such that each edge in graph G
corresponds to the ancestor relation in the tree
Recursive idea of the algorithm:
Repeat until no neighbor of the root is free
• Select a free out-neighbor v of the root and add the edge from root to v to
partial DFS
• Recursively find a DFS’ in graph G restricted to free nodes and node v as the
root’ and add it to partial DFS
root’
root
Lectures on Network Flows
root’’
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Implementing DFS
Structures:
–
–
–
Adjacency list
List (stack) S
Array Discovered[1…n]
Algorithm:
• Set S = {root}
• Consider the top element v in S
–
–
For each out-neighbor w of node v, if w is not Discovered then put w into the stack S and start the next
iteration for w as the top element
Otherwise remove v from the stack, add edge (z,v) to partial DFS, where z is the current top element, and
start the next iteration for z as the top element
Remark: after considering the out-neighbor of node v we remove this neighbor from adjacency list to
avoid considering it many times!
root’
root
Lectures on Network Flows
root’’
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Flows vs. Cuts
(A,B) - cut in graph G:
• A,B is a partition of nodes, s in A, t in B
c(A,B) = ∑e out A c(e) = ∑e in B c(e) is the capacity of this cut
Property:
Minimum cut is equal to the maximum flow
Example:
c(A,B) = 50
u
20
10
30
s
10
20
t
10
30
s
20
v
u
A
10
Lectures on Network Flows
B
t
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Max Flow vs. Min Cut
For any set A containing s we proceed in 3 steps:
• value(f) = ∑v in A ∑e out v f(e) - ∑v in A ∑e in v f(e)
• value(f) = f out(A) - f in(A)
• c(A,B)  f out(A) - f in(A) = value(f)
Conclusion:
Min-cut  value(f)
Example: c(A,B) = 50 and f out(A) = 30
u
20
10
30
s
10
20
t
20
v
u
A
10/10
30/10
s
10/10
20
Lectures on Network Flows
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B
t
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FF-algorithm gives Max-flow
Suppose FF-algorithm stopped with flow f :
• Directed DFS tree rooted in s does not contain t
• It means that cut-capacity between nodes in DFS and
the remaining ones is 0 in residual graph
• It follows that each edge in this cut has been reversed
by augmenting flow, which means that
c(DFS,DFS’) = value(f)
• Using Min-cut  value(f) we get that f is maximum
u
20
u
10
30
s
10
20
v
20
10
Residual
graph:
t
10
20
t
s
DFS
10
2018
Lectures on Network Flows
Min-cut
v
Conclusions
Network flow algorithms:
– Ford-Fulkerson algorithm in time O(mC)
– Correspondence between max-flows and min-cuts
Lectures on Network Flows
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Exercises
READING:
• Chapter 7, Sections 7.1, 7.2, and 7.3
EXERCISES:
• Modify FF-algorithm to work in time
O(m2 log C)
• Find an augmentation scheme to guarantee time
O(mn) in FF-algorithm independently from
integer C
Lectures on Network Flows
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Overview
Previous lecture:
• Network flows
• Ford-Fulkerson algorithm
• Max-flows versus Min-cuts
This lecture:
• Rational and real capacities in network
• Applications: largest matching in bipartite graphs
• Application to disjoint paths problem
Lectures on Network Flows
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Network
Given a directed graph G = (V,E) such that
• each directed edge e has its nonnegative
capacity denoted by ce
• there is a node s (source) with no incoming
edges
• there is a node t (target) with no outgoing
u
edges
20
u,v - internal
nodes
10
30
s
10
t
20
v
Lectures on Network Flows
22
Flow
s-t flow in G = (V,E) is a function f from E to R+
• capacity condition: for each e, 0  f(e)  ce
• conservation condition: for each internal node v,
∑e in v f(e) = ∑e out v f(e)
• there is a node t (target) with no outgoing edges
Property: ∑e in t f(e) = ∑e out s f(e)
Network:
u
20
u
10
30
s
10
t
20
v
Flow:
20
10
s
Lectures on Network Flows
10
10
t
20
v
23
Problem
What is a maximum value f in(t) = f out(s) (flow) for
a given graph G = (V,E) ?
How to compute it efficiently?
Assumption: capacities are positive integers.
Example: f in(t) = f out(s) = 30
Network:
u
20
u
30
s
Flow: 20
10
10
t
20
v
10
s
Lectures on Network Flows
10
10
t
20
v
24
Ford-Fulkerson Algorithm
•
•
Initialize f(e) = 0 for all e
While there is s-t path P in residual graph
–
Augment f through path P and get new f and new residual
graph
Augment f through path P :
• Find minimum capacity on the path
• Go through the path and modify weights
Network: u
20
10
30
s
10
20
v
New
u
u
New
10 residual 20
10
flow: 20
graph:
t
1020 10
10
20
t
s
s
10
10
20
20
v
v 25
Lectures on Network Flows
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Non-integer capacities
•
Rational capacities - rescale them to integers by
multiplying by the common multiply
Real capacities - difficult to handle:
•
–
–
Min-cut = Max-flow
FF-algorithm may work very slowly
Graph: u
10
20
30
s
10
20
v
New
u
u
New
10 residual 20
10
flow: 20
graph:
t
t
10
20
10
10
20
t
s
s
10
10
20
20
v
v
Lectures on Network Flows
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Flows vs. Cuts
(A,B) - cut in graph G:
• A,B is a partition of nodes, s in A, t in B
c(A,B) = ∑e out A c(e) = ∑e in B c(e) is a capacity of the cut
Property:
Minimum cut is equal to the maximum flow
Example:
c(A,B) = 50
u
20
10
30
s
10
20
t
10
30
s
20
v
u
A
10
Lectures on Network Flows
B
t
20
v
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Applications - largest matching
Input: bipartite graph G=(V,W,E)
Goal: largest set of non-incident edges (with different
ends)
Solution using flow algorithms:
• Lets direct edges from V to W, introduce s connected
to all nodes in V, t connected from all nodes in W
• Capacities are 1 for all edges
• Max-flow is the largest matching
s
Lectures on Network Flows
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Applications - disjoint paths
Input: network graph G=(V,E)
Goal: largest set of edge-disjoint paths from s to t
Solution: Using flow algorithms, where each edge
has capacity 1
t
s
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Disjoint paths cont.
Suppose that k is the largest number of edge-disjoint paths
from s to t.
• It is also a flow:
–
–
capacity condition is clear since we push flow with value 1
through each path, and
conservation is satisfied since if a path comes into a node it
also goes out
Conclusion: the largest number of edge-disjoint paths
from s to t is not bigger than Max-flow
t
s
Lectures on Network Flows
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Disjoint paths cont.
Suppose that x is the value of Max-flow produced by FF-algorithm.
How to construct x edge-disjoint paths from s to t ?
By induction on the number of edges in the flow.
For 0 edges trivial (nothing to do, no even a path)
Assume j edges in the flow. Take one of them (s,v) and continue going using
edges in the flow:
•
We go to t - done, since we have path, remove it from the graph and
continue by induction
•
We make a cycle - reduce the flow along the cycle to zero, and the obtained
is a flow having the same value but smaller number of edges - next continue
by induction
s
t
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Complexity
Time of FF-algorithm: O(mn) ( since C = O(n) )
Time of extracting paths: each edge is considered
once while extracting one path, hence total time
O(mn)
Total time: O(mn)
Additional memory: O(m+n) for keeping paths
t
s
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Conclusions
Network flow algorithms:
– Rational capacities are easy to deal with
– Real capacities are difficult to compute although we can alternatively check Min-cut
– Application to the largest matching problem in
time O(mn) (n = C since capacities are 1)
– Application to the edge-disjoint paths problem in
time O(mn) (n = C since capacities are 1)
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Textbook and Exercises
READING:
• Chapter 7, Sections 7.5, 7.6 and 7.7
EXERCISES:
• Find a network with real capacities for which FFalgorithm works very slowly
• Prove formally that FF-algorithm gives the largest
matching in the last application
• Design the algorithm for finding the largest number of
edge-disjoint paths from s to t in undirected network
• Design the algorithm for finding the largest number of
node-disjoint paths from s to t in both directed and
undirected networks
Lectures on Network Flows
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