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Find minimum s-t cut in a flow network
• Difficulty Level : Hard
• Last Updated : 10 Oct, 2020

In a flow network, an s-t cut is a cut that requires the source ‘s’ and the sink ‘t’ to be in different subsets, and it consists of edges going from the source’s side to the sink’s side. The capacity of an s-t cut is defined by the sum of the capacity of each edge in the cut-set. (Source: Wiki)
The problem discussed here is to find minimum capacity s-t cut of the given network. Expected output is all edges of the minimum cut.

For example, in the following flow network, example s-t cuts are {{0 ,1}, {0, 2}}, {{0, 2}, {1, 2}, {1, 3}}, etc. The minimum s-t cut is {{1, 3}, {4, 3}, {4 5}} which has capacity as 12+7+4 = 23.

## Recommended: Please solve it on “PRACTICE” first, before moving on to the solution. We strongly recommend to read the below post first.
Ford-Fulkerson Algorithm for Maximum Flow Problem

Minimum Cut and Maximum Flow
Like Maximum Bipartite Matching, this is another problem which can solved using Ford-Fulkerson Algorithm. This is based on max-flow min-cut theorem.

The max-flow min-cut theorem states that in a flow network, the amount of maximum flow is equal to capacity of the minimum cut. See CLRS book for proof of this theorem.

From Ford-Fulkerson, we get capacity of minimum cut. How to print all edges that form the minimum cut? The idea is to use residual graph.

Following are steps to print all edges of the minimum cut.

1) Run Ford-Fulkerson algorithm and consider the final residual graph.

2) Find the set of vertices that are reachable from the source in the residual graph.

3) All edges which are from a reachable vertex to non-reachable vertex are minimum cut edges. Print all such edges.

Following is the implementation of the above approach.

## C++

 `// C++ program for finding minimum cut using Ford-Fulkerson``#include ``#include ``#include ``#include ``using` `namespace` `std;`` ` `// Number of vertices in given graph``#define V 6`` ` `/* Returns true if there is a path from source 's' to sink 't' in``  ``residual graph. Also fills parent[] to store the path */``int` `bfs(``int` `rGraph[V][V], ``int` `s, ``int` `t, ``int` `parent[])``{``    ``// Create a visited array and mark all vertices as not visited``    ``bool` `visited[V];``    ``memset``(visited, 0, ``sizeof``(visited));`` ` `    ``// Create a queue, enqueue source vertex and mark source vertex``    ``// as visited``    ``queue <``int``> q;``    ``q.push(s);``    ``visited[s] = ``true``;``    ``parent[s] = -1;`` ` `    ``// Standard BFS Loop``    ``while` `(!q.empty())``    ``{``        ``int` `u = q.front();``        ``q.pop();`` ` `        ``for` `(``int` `v=0; v 0)``            ``{``                ``q.push(v);``                ``parent[v] = u;``                ``visited[v] = ``true``;``            ``}``        ``}``    ``}`` ` `    ``// If we reached sink in BFS starting from source, then return``    ``// true, else false``    ``return` `(visited[t] == ``true``);``}`` ` `// A DFS based function to find all reachable vertices from s.  The function``// marks visited[i] as true if i is reachable from s.  The initial values in``// visited[] must be false. We can also use BFS to find reachable vertices``void` `dfs(``int` `rGraph[V][V], ``int` `s, ``bool` `visited[])``{``    ``visited[s] = ``true``;``    ``for` `(``int` `i = 0; i < V; i++)``       ``if` `(rGraph[s][i] && !visited[i])``           ``dfs(rGraph, i, visited);``}`` ` `// Prints the minimum s-t cut``void` `minCut(``int` `graph[V][V], ``int` `s, ``int` `t)``{``    ``int` `u, v;`` ` `    ``// Create a residual graph and fill the residual graph with``    ``// given capacities in the original graph as residual capacities``    ``// in residual graph``    ``int` `rGraph[V][V]; ``// rGraph[i][j] indicates residual capacity of edge i-j``    ``for` `(u = 0; u < V; u++)``        ``for` `(v = 0; v < V; v++)``             ``rGraph[u][v] = graph[u][v];`` ` `    ``int` `parent[V];  ``// This array is filled by BFS and to store path`` ` `    ``// Augment the flow while there is a path from source to sink``    ``while` `(bfs(rGraph, s, t, parent))``    ``{``        ``// Find minimum residual capacity of the edhes along the``        ``// path filled by BFS. Or we can say find the maximum flow``        ``// through the path found.``        ``int` `path_flow = INT_MAX;``        ``for` `(v=t; v!=s; v=parent[v])``        ``{``            ``u = parent[v];``            ``path_flow = min(path_flow, rGraph[u][v]);``        ``}`` ` `        ``// update residual capacities of the edges and reverse edges``        ``// along the path``        ``for` `(v=t; v != s; v=parent[v])``        ``{``            ``u = parent[v];``            ``rGraph[u][v] -= path_flow;``            ``rGraph[v][u] += path_flow;``        ``}``    ``}`` ` `    ``// Flow is maximum now, find vertices reachable from s``    ``bool` `visited[V];``    ``memset``(visited, ``false``, ``sizeof``(visited));``    ``dfs(rGraph, s, visited);`` ` `    ``// Print all edges that are from a reachable vertex to``    ``// non-reachable vertex in the original graph``    ``for` `(``int` `i = 0; i < V; i++)``      ``for` `(``int` `j = 0; j < V; j++)``         ``if` `(visited[i] && !visited[j] && graph[i][j])``              ``cout << i << ``" - "` `<< j << endl;`` ` `    ``return``;``}`` ` `// Driver program to test above functions``int` `main()``{``    ``// Let us create a graph shown in the above example``    ``int` `graph[V][V] = { {0, 16, 13, 0, 0, 0},``                        ``{0, 0, 10, 12, 0, 0},``                        ``{0, 4, 0, 0, 14, 0},``                        ``{0, 0, 9, 0, 0, 20},``                        ``{0, 0, 0, 7, 0, 4},``                        ``{0, 0, 0, 0, 0, 0}``                      ``};`` ` `    ``minCut(graph, 0, 5);`` ` `    ``return` `0;``}`

## Java

 `// Java program for finding min-cut in the given graph``import` `java.util.LinkedList;``import` `java.util.Queue;`` ` `public` `class` `Graph {``         ` `    ``// Returns true if there is a path``    ``// from source 's' to sink 't' in residual ``    ``// graph. Also fills parent[] to store the path ``    ``private` `static` `boolean` `bfs(``int``[][] rGraph, ``int` `s,``                                ``int` `t, ``int``[] parent) {``         ` `        ``// Create a visited array and mark ``        ``// all vertices as not visited     ``        ``boolean``[] visited = ``new` `boolean``[rGraph.length];``         ` `        ``// Create a queue, enqueue source vertex``        ``// and mark source vertex as visited     ``        ``Queue q = ``new` `LinkedList();``        ``q.add(s);``        ``visited[s] = ``true``;``        ``parent[s] = -``1``;``         ` `        ``// Standard BFS Loop     ``        ``while` `(!q.isEmpty()) {``            ``int` `v = q.poll();``            ``for` `(``int` `i = ``0``; i < rGraph.length; i++) {``                ``if` `(rGraph[v][i] > ``0` `&& !visited[i]) {``                    ``q.offer(i);``                    ``visited[i] = ``true``;``                    ``parent[i] = v;``                ``}``            ``}``        ``}``         ` `        ``// If we reached sink in BFS starting ``        ``// from source, then return true, else false     ``        ``return` `(visited[t] == ``true``);``    ``}``     ` `    ``// A DFS based function to find all reachable ``    ``// vertices from s. The function marks visited[i] ``    ``// as true if i is reachable from s. The initial ``    ``// values in visited[] must be false. We can also ``    ``// use BFS to find reachable vertices``    ``private` `static` `void` `dfs(``int``[][] rGraph, ``int` `s,``                                ``boolean``[] visited) {``        ``visited[s] = ``true``;``        ``for` `(``int` `i = ``0``; i < rGraph.length; i++) {``                ``if` `(rGraph[s][i] > ``0` `&& !visited[i]) {``                    ``dfs(rGraph, i, visited);``                ``}``        ``}``    ``}`` ` `    ``// Prints the minimum s-t cut``    ``private` `static` `void` `minCut(``int``[][] graph, ``int` `s, ``int` `t) {``        ``int` `u,v;``         ` `        ``// Create a residual graph and fill the residual ``        ``// graph with given capacities in the original ``        ``// graph as residual capacities in residual graph``        ``// rGraph[i][j] indicates residual capacity of edge i-j``        ``int``[][] rGraph = ``new` `int``[graph.length][graph.length]; ``        ``for` `(``int` `i = ``0``; i < graph.length; i++) {``            ``for` `(``int` `j = ``0``; j < graph.length; j++) {``                ``rGraph[i][j] = graph[i][j];``            ``}``        ``}`` ` `        ``// This array is filled by BFS and to store path``        ``int``[] parent = ``new` `int``[graph.length]; ``         ` `        ``// Augment the flow while tere is path from source to sink     ``        ``while` `(bfs(rGraph, s, t, parent)) {``             ` `            ``// Find minimum residual capacity of the edhes ``            ``// along the path filled by BFS. Or we can say ``            ``// find the maximum flow through the path found.``            ``int` `pathFlow = Integer.MAX_VALUE;         ``            ``for` `(v = t; v != s; v = parent[v]) {``                ``u = parent[v];``                ``pathFlow = Math.min(pathFlow, rGraph[u][v]);``            ``}``             ` `            ``// update residual capacities of the edges and ``            ``// reverse edges along the path``            ``for` `(v = t; v != s; v = parent[v]) {``                ``u = parent[v];``                ``rGraph[u][v] = rGraph[u][v] - pathFlow;``                ``rGraph[v][u] = rGraph[v][u] + pathFlow;``            ``}``        ``}``         ` `        ``// Flow is maximum now, find vertices reachable from s     ``        ``boolean``[] isVisited = ``new` `boolean``[graph.length];     ``        ``dfs(rGraph, s, isVisited);``         ` `        ``// Print all edges that are from a reachable vertex to``        ``// non-reachable vertex in the original graph     ``        ``for` `(``int` `i = ``0``; i < graph.length; i++) {``            ``for` `(``int` `j = ``0``; j < graph.length; j++) {``                ``if` `(graph[i][j] > ``0` `&& isVisited[i] && !isVisited[j]) {``                    ``System.out.println(i + ``" - "` `+ j);``                ``}``            ``}``        ``}``    ``}`` ` `    ``//Driver Program``    ``public` `static` `void` `main(String args[]) {``         ` `        ``// Let us create a graph shown in the above example``        ``int` `graph[][] = { {``0``, ``16``, ``13``, ``0``, ``0``, ``0``},``                ``{``0``, ``0``, ``10``, ``12``, ``0``, ``0``},``                ``{``0``, ``4``, ``0``, ``0``, ``14``, ``0``},``                ``{``0``, ``0``, ``9``, ``0``, ``0``, ``20``},``                ``{``0``, ``0``, ``0``, ``7``, ``0``, ``4``},``                ``{``0``, ``0``, ``0``, ``0``, ``0``, ``0``}``            ``};``        ``minCut(graph, ``0``, ``5``);``    ``}``}``// This code is contributed by Himanshu Shekhar`

## Python

 `# Python program for finding min-cut in the given graph ``# Complexity : (E*(V^3)) ``# Total augmenting path = VE and BFS ``# with adj matrix takes :V^2 times `` ` `from` `collections ``import` `defaultdict `` ` `# This class represents a directed graph``# using adjacency matrix representation ``class` `Graph: `` ` `    ``def` `__init__(``self``,graph): ``        ``self``.graph ``=` `graph ``# residual graph ``        ``self``.org_graph ``=` `[i[:] ``for` `i ``in` `graph] ``        ``self``. ROW ``=` `len``(graph) ``        ``self``.COL ``=` `len``(graph[``0``]) `` ` ` ` `    ``'''Returns true if there is a path from ``    ``source 's' to sink 't' in ``    ``residual graph. Also fills ``    ``parent[] to store the path '''``    ``def` `BFS(``self``,s, t, parent): `` ` `        ``# Mark all the vertices as not visited ``        ``visited ``=``[``False``]``*``(``self``.ROW) `` ` `        ``# Create a queue for BFS ``        ``queue``=``[] `` ` `        ``# Mark the source node as visited and enqueue it ``        ``queue.append(s) ``        ``visited[s] ``=` `True`` ` `        ``# Standard BFS Loop ``        ``while` `queue: `` ` `            ``#Dequeue a vertex from queue and print it ``            ``u ``=` `queue.pop(``0``) `` ` `            ``# Get all adjacent vertices of ``            ``# the dequeued vertex u ``            ``# If a adjacent has not been``            ``# visited, then mark it ``            ``# visited and enqueue it ``            ``for` `ind, val ``in` `enumerate``(``self``.graph[u]): ``                ``if` `visited[ind] ``=``=` `False` `and` `val > ``0` `: ``                    ``queue.append(ind) ``                    ``visited[ind] ``=` `True``                    ``parent[ind] ``=` `u `` ` `        ``# If we reached sink in BFS starting``        ``# from source, then return ``        ``# true, else false ``        ``return` `True` `if` `visited[t] ``else` `False``         ` `    ``# Function for Depth first search ``    ``# Traversal of the graph``    ``def` `dfs(``self``, graph,s,visited):``        ``visited[s]``=``True``        ``for` `i ``in` `range``(``len``(graph)):``            ``if` `graph[s][i]>``0` `and` `not` `visited[i]:``                ``self``.dfs(graph,i,visited)`` ` `    ``# Returns the min-cut of the given graph ``    ``def` `minCut(``self``, source, sink): `` ` `        ``# This array is filled by BFS and to store path ``        ``parent ``=` `[``-``1``]``*``(``self``.ROW) `` ` `        ``max_flow ``=` `0` `# There is no flow initially `` ` `        ``# Augment the flow while there is path from source to sink ``        ``while` `self``.BFS(source, sink, parent) : `` ` `            ``# Find minimum residual capacity of the edges along the ``            ``# path filled by BFS. Or we can say find the maximum flow ``            ``# through the path found. ``            ``path_flow ``=` `float``(``"Inf"``) ``            ``s ``=` `sink ``            ``while``(s !``=` `source): ``                ``path_flow ``=` `min` `(path_flow, ``self``.graph[parent[s]][s]) ``                ``s ``=` `parent[s] `` ` `            ``# Add path flow to overall flow ``            ``max_flow ``+``=` `path_flow `` ` `            ``# update residual capacities of the edges and reverse edges ``            ``# along the path ``            ``v ``=` `sink ``            ``while``(v !``=` `source): ``                ``u ``=` `parent[v] ``                ``self``.graph[u][v] ``-``=` `path_flow ``                ``self``.graph[v][u] ``+``=` `path_flow ``                ``v ``=` `parent[v] `` ` `        ``visited``=``len``(``self``.graph)``*``[``False``]``        ``self``.dfs(``self``.graph,s,visited)`` ` `        ``# print the edges which initially had weights ``        ``# but now have 0 weight ``        ``for` `i ``in` `range``(``self``.ROW): ``            ``for` `j ``in` `range``(``self``.COL): ``                ``if` `self``.graph[i][j] ``=``=` `0` `and``\``                ``self``.org_graph[i][j] > ``0` `and` `visited[i]: ``                    ``print` `str``(i) ``+` `" - "` `+` `str``(j) `` ` ` ` `# Create a graph given in the above diagram ``graph ``=` `[[``0``, ``16``, ``13``, ``0``, ``0``, ``0``], ``        ``[``0``, ``0``, ``10``, ``12``, ``0``, ``0``], ``        ``[``0``, ``4``, ``0``, ``0``, ``14``, ``0``], ``        ``[``0``, ``0``, ``9``, ``0``, ``0``, ``20``], ``        ``[``0``, ``0``, ``0``, ``7``, ``0``, ``4``], ``        ``[``0``, ``0``, ``0``, ``0``, ``0``, ``0``]] `` ` `g ``=` `Graph(graph) `` ` `source ``=` `0``; sink ``=` `5`` ` `g.minCut(source, sink) `` ` `# This code is contributed by Neelam Yadav `

## C#

 `// C# program for finding min-cut in the given graph``using` `System;``using` `System.Collections.Generic;`` ` `class` `Graph``{``         ` `    ``// Returns true if there is a path``    ``// from source 's' to sink 't' in residual ``    ``// graph. Also fills parent[] to store the path ``    ``private` `static` `bool` `bfs(``int``[,] rGraph, ``int` `s,``                            ``int` `t, ``int``[] parent) ``    ``{``         ` `        ``// Create a visited array and mark ``        ``// all vertices as not visited     ``        ``bool``[] visited = ``new` `bool``[rGraph.Length];``         ` `        ``// Create a queue, enqueue source vertex``        ``// and mark source vertex as visited     ``        ``Queue<``int``> q = ``new` `Queue<``int``>();``        ``q.Enqueue(s);``        ``visited[s] = ``true``;``        ``parent[s] = -1;``         ` `        ``// Standard BFS Loop     ``        ``while` `(q.Count != 0) ``        ``{``            ``int` `v = q.Dequeue();``            ``for` `(``int` `i = 0; i < rGraph.GetLength(0); i++) ``            ``{``                ``if` `(rGraph[v,i] > 0 && !visited[i]) ``                ``{``                    ``q.Enqueue(i);``                    ``visited[i] = ``true``;``                    ``parent[i] = v;``                ``}``            ``}``        ``}``         ` `        ``// If we reached sink in BFS starting ``        ``// from source, then return true, else false     ``        ``return` `(visited[t] == ``true``);``    ``}``     ` `    ``// A DFS based function to find all reachable ``    ``// vertices from s. The function marks visited[i] ``    ``// as true if i is reachable from s. The initial ``    ``// values in visited[] must be false. We can also ``    ``// use BFS to find reachable vertices``    ``private` `static` `void` `dfs(``int``[,] rGraph, ``int` `s,``                            ``bool``[] visited) ``    ``{``        ``visited[s] = ``true``;``        ``for` `(``int` `i = 0; i < rGraph.GetLength(0); i++) ``        ``{``            ``if` `(rGraph[s,i] > 0 && !visited[i])``            ``{``                ``dfs(rGraph, i, visited);``            ``}``        ``}``    ``}`` ` `    ``// Prints the minimum s-t cut``    ``private` `static` `void` `minCut(``int``[,] graph, ``int` `s, ``int` `t) ``    ``{``        ``int` `u, v;``         ` `        ``// Create a residual graph and fill the residual ``        ``// graph with given capacities in the original ``        ``// graph as residual capacities in residual graph``        ``// rGraph[i,j] indicates residual capacity of edge i-j``        ``int``[,] rGraph = ``new` `int``[graph.Length,graph.Length]; ``        ``for` `(``int` `i = 0; i < graph.GetLength(0); i++)``        ``{``            ``for` `(``int` `j = 0; j < graph.GetLength(1); j++)``            ``{``                ``rGraph[i, j] = graph[i, j];``            ``}``        ``}`` ` `        ``// This array is filled by BFS and to store path``        ``int``[] parent = ``new` `int``[graph.Length]; ``         ` `        ``// Augment the flow while there is path``        ``// from source to sink     ``        ``while` `(bfs(rGraph, s, t, parent)) ``        ``{``             ` `            ``// Find minimum residual capacity of the edhes ``            ``// along the path filled by BFS. Or we can say ``            ``// find the maximum flow through the path found.``            ``int` `pathFlow = ``int``.MaxValue;         ``            ``for` `(v = t; v != s; v = parent[v]) ``            ``{``                ``u = parent[v];``                ``pathFlow = Math.Min(pathFlow, rGraph[u, v]);``            ``}``             ` `            ``// update residual capacities of the edges and ``            ``// reverse edges along the path``            ``for` `(v = t; v != s; v = parent[v])``            ``{``                ``u = parent[v];``                ``rGraph[u, v] = rGraph[u, v] - pathFlow;``                ``rGraph[v, u] = rGraph[v, u] + pathFlow;``            ``} ``        ``}``         ` `        ``// Flow is maximum now, find vertices reachable from s     ``        ``bool``[] isVisited = ``new` `bool``[graph.Length];     ``        ``dfs(rGraph, s, isVisited);``         ` `        ``// Print all edges that are from a reachable vertex to``        ``// non-reachable vertex in the original graph     ``        ``for` `(``int` `i = 0; i < graph.GetLength(0); i++) ``        ``{``            ``for` `(``int` `j = 0; j < graph.GetLength(1); j++)``            ``{``                ``if` `(graph[i, j] > 0 && ``                    ``isVisited[i] && !isVisited[j])``                ``{``                    ``Console.WriteLine(i + ``" - "` `+ j);``                ``}``            ``}``        ``}``    ``}`` ` `    ``// Driver Code``    ``public` `static` `void` `Main(String []args)``    ``{``         ` `        ``// Let us create a graph shown ``        ``// in the above example``        ``int` `[,]graph = {{0, 16, 13, 0, 0, 0},``                        ``{0, 0, 10, 12, 0, 0},``                        ``{0, 4, 0, 0, 14, 0},``                        ``{0, 0, 9, 0, 0, 20},``                        ``{0, 0, 0, 7, 0, 4},``                        ``{0, 0, 0, 0, 0, 0}};``        ``minCut(graph, 0, 5);``    ``}``}`` ` `// This code is contributed by PrinciRaj1992`

Output:
```1 - 3
4 - 3
4 - 5```

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