Given an undirected graph and a set of vertices, we have to count the number of non-reachable nodes from the given head node using a depth-first search.

Consider below undirected graph with two disconnected components:

In this graph, if we consider 0 as a head node, then the node 0, 1 and 2 are reachable. We mark all the reachable nodes as visited. All those nodes which are not marked as visited i.e, node 3 and 4 are non-reachable nodes. Hence their count is 2.

**Example:**

Input : 5 0 1 0 2 1 2 3 4 Output : 2

We can either use BFS or DFS for this purpose. In the below implementation, DFS is used. We do DFS from a given source. Since the given graph is undirected, all the vertices that belong to the disconnected component are non-reachable nodes. We use the visited array for this purpose, the array which is used to keep track of non-visited vertices in DFS. In DFS, if we start from the head node it will mark all the nodes connected to the head node as visited. Then after traversing the graph, we count the number of nodes that are not marked as visited from the head node.

## C++

`// C++ program to count non-reachable nodes ` `// from a given source using DFS. ` `#include <iostream> ` `#include <list> ` `using` `namespace` `std; ` ` ` `// Graph class represents a directed graph ` `// using adjacency list representation ` `class` `Graph { ` ` ` `int` `V; ` `// No. of vertices ` ` ` ` ` `// Pointer to an array containing ` ` ` `// adjacency lists ` ` ` `list<` `int` `>* adj; ` ` ` ` ` `// A recursive function used by DFS ` ` ` `void` `DFSUtil(` `int` `v, ` `bool` `visited[]); ` ` ` `public` `: ` ` ` `Graph(` `int` `V); ` `// Constructor ` ` ` ` ` `// function to add an edge to graph ` ` ` `void` `addEdge(` `int` `v, ` `int` `w); ` ` ` ` ` `// DFS traversal of the vertices ` ` ` `// reachable from v ` ` ` `int` `countNotReach(` `int` `v); ` `}; ` ` ` `Graph::Graph(` `int` `V) ` `{ ` ` ` `this` `->V = V; ` ` ` `adj = ` `new` `list<` `int` `>[V]; ` `} ` ` ` `void` `Graph::addEdge(` `int` `v, ` `int` `w) ` `{ ` ` ` `adj[v].push_back(w); ` `// Add w to v’s list. ` ` ` `adj[w].push_back(v); ` `// Add v to w's list. ` `} ` ` ` `void` `Graph::DFSUtil(` `int` `v, ` `bool` `visited[]) ` `{ ` ` ` `// Mark the current node as visited and ` ` ` `// print it ` ` ` `visited[v] = ` `true` `; ` ` ` ` ` `// Recur for all the vertices adjacent ` ` ` `// to this vertex ` ` ` `list<` `int` `>::iterator i; ` ` ` `for` `(i = adj[v].begin(); i != adj[v].end(); ++i) ` ` ` `if` `(!visited[*i]) ` ` ` `DFSUtil(*i, visited); ` `} ` ` ` `// Returns count of not reachable nodes from ` `// vertex v. ` `// It uses recursive DFSUtil() ` `int` `Graph::countNotReach(` `int` `v) ` `{ ` ` ` `// Mark all the vertices as not visited ` ` ` `bool` `* visited = ` `new` `bool` `[V]; ` ` ` `for` `(` `int` `i = 0; i < V; i++) ` ` ` `visited[i] = ` `false` `; ` ` ` ` ` `// Call the recursive helper function ` ` ` `// to print DFS traversal ` ` ` `DFSUtil(v, visited); ` ` ` ` ` `// Return count of not visited nodes ` ` ` `int` `count = 0; ` ` ` `for` `(` `int` `i = 0; i < V; i++) { ` ` ` `if` `(visited[i] == ` `false` `) ` ` ` `count++; ` ` ` `} ` ` ` `return` `count; ` `} ` ` ` `int` `main() ` `{ ` ` ` `// Create a graph given in the above diagram ` ` ` `Graph g(8); ` ` ` `g.addEdge(0, 1); ` ` ` `g.addEdge(0, 2); ` ` ` `g.addEdge(1, 2); ` ` ` `g.addEdge(3, 4); ` ` ` `g.addEdge(4, 5); ` ` ` `g.addEdge(6, 7); ` ` ` ` ` `cout << g.countNotReach(2); ` ` ` ` ` `return` `0; ` `} ` |

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## Python3

`# Python3 program to count non-reachable ` `# nodes from a given source using DFS. ` ` ` `# Graph class represents a directed graph ` `# using adjacency list representation ` `class` `Graph: ` ` ` `def` `__init__(` `self` `, V): ` ` ` `self` `.V ` `=` `V ` ` ` `self` `.adj ` `=` `[[] ` `for` `i ` `in` `range` `(V)] ` ` ` ` ` `def` `addEdge(` `self` `, v, w): ` ` ` `self` `.adj[v].append(w) ` `# Add w to v’s list. ` ` ` `self` `.adj[w].append(v) ` `# Add v to w's list. ` ` ` ` ` `def` `DFSUtil(` `self` `, v, visited): ` ` ` ` ` `# Mark the current node as ` ` ` `# visited and print it ` ` ` `visited[v] ` `=` `True` ` ` ` ` `# Recur for all the vertices ` ` ` `# adjacent to this vertex ` ` ` `i ` `=` `self` `.adj[v][` `0` `] ` ` ` `for` `i ` `in` `self` `.adj[v]: ` ` ` `if` `(` `not` `visited[i]): ` ` ` `self` `.DFSUtil(i, visited) ` ` ` ` ` `# Returns count of not reachable ` ` ` `# nodes from vertex v. ` ` ` `# It uses recursive DFSUtil() ` ` ` `def` `countNotReach(` `self` `, v): ` ` ` ` ` `# Mark all the vertices as not visited ` ` ` `visited ` `=` `[` `False` `] ` `*` `self` `.V ` ` ` ` ` `# Call the recursive helper ` ` ` `# function to prDFS traversal ` ` ` `self` `.DFSUtil(v, visited) ` ` ` ` ` `# Return count of not visited nodes ` ` ` `count ` `=` `0` ` ` `for` `i ` `in` `range` `(` `self` `.V): ` ` ` `if` `(visited[i] ` `=` `=` `False` `): ` ` ` `count ` `+` `=` `1` ` ` `return` `count ` ` ` `# Driver Code ` `if` `__name__ ` `=` `=` `'__main__'` `: ` ` ` ` ` `# Create a graph given in the ` ` ` `# above diagram ` ` ` `g ` `=` `Graph(` `8` `) ` ` ` `g.addEdge(` `0` `, ` `1` `) ` ` ` `g.addEdge(` `0` `, ` `2` `) ` ` ` `g.addEdge(` `1` `, ` `2` `) ` ` ` `g.addEdge(` `3` `, ` `4` `) ` ` ` `g.addEdge(` `4` `, ` `5` `) ` ` ` `g.addEdge(` `6` `, ` `7` `) ` ` ` ` ` `print` `(g.countNotReach(` `2` `)) ` ` ` `# This code is contributed by PranchalK ` |

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**Output:**

5

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