## Detect cycle in undirected graph

Given an undirected graph, find if there is a cycle in that undirected graph. For example, below graph has cycles as 2->3->4->2 and 5->4->6->5 and a few more. A simple definition of a cycle in an undirected graph would be: If while traversing the graph, we reach a node which we have already traversed to reach the current node, then there is a cycle in the graph.

How can we detect the above condition? It’s an application of Depth First Search. Do a depth-first traversal of a graph. In depth first traversal, we visit the current node and go deep to one of its unvisited neighbors. If a neighbor of the current node is already visited, that means there is a possibility of a cycle. However, make sure that the already visited neighbor is not the parent of the current node, otherwise, there will be always a cycle of two nodes in an undirected graph.

While depth first traversing a graph, keep track of the nodes which are on the stack at present. If there is an edge from the current node to any one of the nodes which are on recursion stack, we say there is a cycle in undirected graph.

To avoid additional space, we can pass the parent pointer to the recursive call and check if the visited node is the parent.

#### Detect cycle in undirected graph: implementation

```
package com.company.Graphs;

import java.util.*;

/**
* Created by sangar on 21.12.18.
*/
private Map<Integer, ArrayList<Integer>> G;
private boolean isDirected;
private int count;

this.G = new HashMap<>();
this.isDirected = isDirected;
}

public void addEdge(int start, int dest){

if(this.G.containsKey(start)){
}else{
this.G.put(start, new ArrayList<>(Arrays.asList(dest)));
}

if(!this.G.containsKey(dest)) {
this.G.put(dest, new ArrayList<>());
}
//In case graph is undirected
if(!this.isDirected) {
}
}

public boolean isEdge(int start, int dest){
if(this.G.containsKey(start)){
return this.G.get(start).contains(dest);
}

return false;
}

private boolean isCycleUtil(int u, boolean[] visited, int parent){

for(int v: this.G.get(u)) {
if(!visited[v]){
visited[v] = true;
return isCycleUtil(v, visited, u);
}
else if(v != parent){
return true;
}
}

return false;
}

public boolean isCycle(){
boolean[] visited = new boolean[this.G.size() + 1];

for(int i = 1; i < this.G.size() + 1; i++) {
if (!visited[i]) {
return isCycleUtil(i, visited, -1);
}
}
return false;
}
}
```

The complexity of the DFS approach to finding a cycle in an undirected graph is `O(V+E)` where V is the number of vertices and E is the number of edges.

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## Depth first traversal of a graph

In the last post, we discussed how to represent a graph in a programmatic way using adjacency matrix and adjacency list representation. Let’s discuss now, how to traverse a graph. There are two ways to traverse a graph: Depth First traversal, commonly called DFS and Breadth First traversal, commonly called as BFS.

### What is Depth First Traversal (DFS) ?

Depth First search or traversal is a technique in which we go as deep in the graph as possible and explore nodes, that is we move down the graph from one node to another till we can. Once we reach a node where we cannot go further down, we backtrack and move to node one before and so on.
A depth-first traversal of a graph is a very good example to understand how backtracking works.

Let’s see how do we traverse a graph depth first.

2. If u is already visited, return.
3. Mark `visited[u] = true`.
4. For all neighbors v of u.
5. v is now new u, so `u=v` and go back to step 2
6. When all neighbors are processed, backtrack to parent of current u.

When we reach at start node S again and there is no edge to be explored at S, that would be the end of traversal.

Let’s take an example and see how it works. Given the graph below, let’s trace how depth first search will go. We start with `node(1)`, and mark it as visited. Now, we pick neighbor of `node(1)`, and go down to `node(2)` and mar it as visited too. Next, we move to the neighbor of `node(2)`, which is `node(3)`. After marking `node(3)` visited, we move to `node(4)`. Next, we move to `node(6)`. At this point, we have no neighbors to visit from `node(6)`. So, we start backtracking. At `node(2)`, we have another neighbor called `node(4)`, however, node(4) is already visited when we went deep from node(3). So nothing happens, we move further back. At `node(1)`, we have neighbor node(5).
Since `node(5)` is not visited, we mark it as visited. At this point, there is no node with neighbor not traversed, so end of depth-first traversal.

#### Depth first traversal : implementation

```package com.company.Graphs;

import java.util.ArrayList;

/**
* Created by sangar on 21.12.18.
*/
private boolean [][] G;
private boolean isDirected;

this.G  = new boolean[numNodes+1][numNodes+1];
this.isDirected = isDirected;
}

public void addEdge(int start, int dest){
this.G[start][dest] = true;
if(!isDirected){
this.G[dest][start] = true;
}
}

public boolean isEdge(int start, int dest){
return this.G[start][dest];
}

private void DFS(int node, boolean[] visited,
ArrayList<Integer> traversal){
if(!visited[node]){
visited[node] = true;

//Complexity of this loop is O(V * E)
for(int i=1; i< this.G.length; i++){
if(this.G[node][i]){
//This will be called E times (number of edges)
DFS(i, visited, traversal);
}
}
}
}
public ArrayList<Integer> depthFirstTraversal(){

boolean[] visited = new boolean[this.G.length];
ArrayList<Integer> traversal = new ArrayList<>();

DFS(1, visited, traversal);

System.out.println(traversal);
return traversal;

}
}

```

The complexity of the above implementation is `O(V * E)`.

Depth first traversal when the graph is represented using adjacency list.

```package com.company.Graphs;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;

/**
* Created by sangar on 21.12.18.
*/
private Map<Integer, ArrayList<Integer>> G;
boolean isDirected;

this.G = new HashMap<>();
this.isDirected = isDirected;
}

public void addEdge(int start, int dest){

if(this.G.containsKey(start)){
}else{
this.G.put(start, new ArrayList<>(Arrays.asList(dest)));
}

//In case graph is undirected
if(!this.isDirected) {
if (this.G.containsKey(dest)) {
} else {
this.G.put(dest, new ArrayList<>(Arrays.asList(start)));
}
}
}

public boolean isEdge(int start, int dest){
if(this.G.containsKey(start)){
return this.G.get(start).contains(dest);
}

return false;
}

private void DFS(int node, boolean[] visited,
ArrayList<Integer> traversal){
if(!visited[node]){
visited[node] = true;

this.G.get(node).forEach(v -> DFS(v, visited, traversal));
}
}
public ArrayList<Integer> depthFirstTraversal(){

boolean[] visited = new boolean[this.G.size()+1];
ArrayList<Integer> traversal = new ArrayList<>();

DFS(1, visited, traversal);

System.out.println(traversal);
return traversal;

}
}

```

The complexity of the above implementation is `O(V + E)`.

Both above implementations are recursive implementation. We can use a stack to replace recursion. Algorithm remains the same, for each node visited, put all the adjacent nodes into the stack. Take next node from the stack, mark it as visited and put all the adjacent nodes to stack. Repeat the process till stack is not empty.

```public ArrayList<Integer> depthFirstTraversalNonRecursive(){

boolean[] visited = new boolean[this.G.length];
ArrayList<Integer> traversal = new ArrayList<>();

Stack s = new Stack();
s.push(1);
visited = true;

while(!s.isEmpty()){
int currentNode = (int)s.pop();

for(int i=1; i< this.G.length; i++){
if(this.G[currentNode][i] && !visited[i]){
s.push(i);
visited[i] = true;
}
}
}

System.out.println(traversal);
return traversal;

}
```

This implementation of dept first traversal or DFS works only if the graph has only one connected component. If there are multiple connected components in the graph, this will not work. For example, above implementation will not work for below graph.

It can be easily fixed. All we need to do is that we try to start DFS from each node of the graph. Check if a node is not already visited for earlier DFS run, then start another DFS from that node. Below is the changed code.

```  private void DFS(int node, boolean[] visited,
ArrayList<Integer> traversal){
if(!visited[node]){
visited[node] = true;

for(int i=1; i< this.G.length; i++){
if(this.G[node][i]){
DFS(i, visited, traversal);
}
}
}
}
public ArrayList<Integer> depthFirstTraversal(){

boolean[] visited = new boolean[this.G.length];
ArrayList<Integer> traversal = new ArrayList<>();

for(int i=1; i<this.G.length; i++) {
if(!visited[i])
DFS(i, visited, traversal);
}

System.out.println(traversal);
return traversal;
}
```

### Applications of DFS

1. Minimum spanning tree
2. To check if graph has a cycle.
3. Topological sorting
4. To find strongly connected components of graph
5. To find bridges in graph

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