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Java/DataStructures/Graphs/BellmanFord.java at dev · primary-code-test/Java · GitHub
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DataStructures
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Graphs
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BellmanFord.java
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DataStructures
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Graphs
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BellmanFord.java
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package
DataStructures
.
Graphs
;
import
java
.
util
.*;
class
BellmanFord
/*Implementation of Bellman ford to detect negative cycles. Graph accepts inputs in form of edges which have
start vertex, end vertes and weights. Vertices should be labelled with a number between 0 and total number of vertices-1,both inclusive*/
{
int
vertex
,
edge
;
private
Edge
edges
[];
private
int
index
=
0
;
BellmanFord
(
int
v
,
int
e
) {
vertex
=
v
;
edge
=
e
;
edges
=
new
Edge
[
e
];
}
class
Edge
{
int
u
,
v
;
int
w
;
/**
* @param u Source Vertex
* @param v End vertex
* @param c Weight
*/
public
Edge
(
int
a
,
int
b
,
int
c
) {
u
=
a
;
v
=
b
;
w
=
c
;
}
}
/**
* @param p[] Parent array which shows updates in edges
* @param i Current vertex under consideration
*/
void
printPath
(
int
p
[],
int
i
) {
if
(
p
[
i
] == -
1
)
// Found the path back to parent
return
;
printPath
(
p
,
p
[
i
]);
System
.
out
.
print
(
i
+
" "
);
}
public
static
void
main
(
String
args
[]) {
BellmanFord
obj
=
new
BellmanFord
(
0
,
0
);
// Dummy object to call nonstatic variables
obj
.
go
();
}
public
void
go
()
// Interactive run for understanding the class first time. Assumes source vertex is 0 and
// shows distaance to all vertices
{
Scanner
sc
=
new
Scanner
(
System
.
in
);
// Grab scanner object for user input
int
i
,
v
,
e
,
u
,
ve
,
w
,
j
,
neg
=
0
;
System
.
out
.
println
(
"Enter no. of vertices and edges please"
);
v
=
sc
.
nextInt
();
e
=
sc
.
nextInt
();
Edge
arr
[] =
new
Edge
[
e
];
// Array of edges
System
.
out
.
println
(
"Input edges"
);
for
(
i
=
0
;
i
<
e
;
i
++) {
u
=
sc
.
nextInt
();
ve
=
sc
.
nextInt
();
w
=
sc
.
nextInt
();
arr
[
i
] =
new
Edge
(
u
,
ve
,
w
);
}
int
dist
[] =
new
int
[
v
];
// Distance array for holding the finalized shortest path distance between source
// and all vertices
int
p
[] =
new
int
[
v
];
// Parent array for holding the paths
for
(
i
=
0
;
i
<
v
;
i
++)
dist
[
i
] =
Integer
.
MAX_VALUE
;
// Initializing distance values
dist
[
0
] =
0
;
p
[
0
] = -
1
;
for
(
i
=
0
;
i
<
v
-
1
;
i
++) {
for
(
j
=
0
;
j
<
e
;
j
++) {
if
((
int
)
dist
[
arr
[
j
].
u
] !=
Integer
.
MAX_VALUE
&&
dist
[
arr
[
j
].
v
] >
dist
[
arr
[
j
].
u
] +
arr
[
j
].
w
) {
dist
[
arr
[
j
].
v
] =
dist
[
arr
[
j
].
u
] +
arr
[
j
].
w
;
// Update
p
[
arr
[
j
].
v
] =
arr
[
j
].
u
;
}
}
}
// Final cycle for negative checking
for
(
j
=
0
;
j
<
e
;
j
++)
if
((
int
)
dist
[
arr
[
j
].
u
] !=
Integer
.
MAX_VALUE
&&
dist
[
arr
[
j
].
v
] >
dist
[
arr
[
j
].
u
] +
arr
[
j
].
w
) {
neg
=
1
;
System
.
out
.
println
(
"Negative cycle"
);
break
;
}
if
(
neg
==
0
)
// Go ahead and show results of computaion
{
System
.
out
.
println
(
"Distances are: "
);
for
(
i
=
0
;
i
<
v
;
i
++)
System
.
out
.
println
(
i
+
" "
+
dist
[
i
]);
System
.
out
.
println
(
"Path followed:"
);
for
(
i
=
0
;
i
<
v
;
i
++) {
System
.
out
.
print
(
"0 "
);
printPath
(
p
,
i
);
System
.
out
.
println
();
}
}
sc
.
close
();
}
/**
* @param source Starting vertex
* @param end Ending vertex
* @param Edge Array of edges
*/
public
void
show
(
int
source
,
int
end
,
Edge
arr
[])
// Just shows results of computation, if graph is passed to it. The graph should
// be created by using addEdge() method and passed by calling getEdgeArray() method
{
int
i
,
j
,
v
=
vertex
,
e
=
edge
,
neg
=
0
;
double
dist
[] =
new
double
[
v
];
// Distance array for holding the finalized shortest path distance between source
// and all vertices
int
p
[] =
new
int
[
v
];
// Parent array for holding the paths
for
(
i
=
0
;
i
<
v
;
i
++)
dist
[
i
] =
Integer
.
MAX_VALUE
;
// Initializing distance values
dist
[
source
] =
0
;
p
[
source
] = -
1
;
for
(
i
=
0
;
i
<
v
-
1
;
i
++) {
for
(
j
=
0
;
j
<
e
;
j
++) {
if
((
int
)
dist
[
arr
[
j
].
u
] !=
Integer
.
MAX_VALUE
&&
dist
[
arr
[
j
].
v
] >
dist
[
arr
[
j
].
u
] +
arr
[
j
].
w
) {
dist
[
arr
[
j
].
v
] =
dist
[
arr
[
j
].
u
] +
arr
[
j
].
w
;
// Update
p
[
arr
[
j
].
v
] =
arr
[
j
].
u
;
}
}
}
// Final cycle for negative checking
for
(
j
=
0
;
j
<
e
;
j
++)
if
((
int
)
dist
[
arr
[
j
].
u
] !=
Integer
.
MAX_VALUE
&&
dist
[
arr
[
j
].
v
] >
dist
[
arr
[
j
].
u
] +
arr
[
j
].
w
) {
neg
=
1
;
System
.
out
.
println
(
"Negative cycle"
);
break
;
}
if
(
neg
==
0
)
// Go ahead and show results of computaion
{
System
.
out
.
println
(
"Distance is: "
+
dist
[
end
]);
System
.
out
.
println
(
"Path followed:"
);
System
.
out
.
print
(
source
+
" "
);
printPath
(
p
,
end
);
System
.
out
.
println
();
}
}
/**
* @param x Source Vertex
* @param y End vertex
* @param z Weight
*/
public
void
addEdge
(
int
x
,
int
y
,
int
z
)
// Adds unidirectionl Edge
{
edges
[
index
++] =
new
Edge
(
x
,
y
,
z
);
}
public
Edge
[]
getEdgeArray
() {
return
edges
;
}
}
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