# Number of cyclic elements in an array where we can jump according to value

Given a array arr[] of n integers. For every value arr[i], we can move to arr[i] + 1 clockwise
considering array elements in cycle. We need to count cyclic elements in the array. An element is cyclic if starting from it and moving to arr[i] + 1 leads to same element.

Examples:

```Input : arr[] = {1, 1, 1, 1}
Output : 4
All 4 elements are cyclic elements.
1 -> 3 -> 1
2 -> 4 -> 2
3 -> 1 -> 3
4 -> 2 -> 4

Input : arr[] = {3, 0, 0, 0}
Output : 1
There is one cyclic point 1,
1 -> 1
The path covered starting from 2 is
2 -> 3 -> 4 -> 1 -> 1.

The path covered starting from 3 is
2 -> 3 -> 4 -> 1 -> 1.

The path covered starting from 4 is
4 -> 1 -> 1
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

One simple solution is to check all elements one by one. We follow simple path starting from every element arr[i], we go to arr[i] + 1. If we come back to a visited element other than arr[i], we do not count arr[i]. Time complexity of this solution is O(n2)

An efficient solution is based on below steps.
1) Create a directed graph using array indexes as nodes. We add an edge from i to node (arr[i] + 1)%n.
2) Once a graph is created we find all strongly connected components using Kosaraju’s Algorithm
3) We finally return sum of counts of nodes in individual strongly connected component.

 `// CPP program to count cyclic points ` `// in an array using Kosaraju's Algorithm ` `#include ` `using` `namespace` `std; ` ` `  `// Most of the code is taken from below link ` `// https://tutorialspoint.dev/slugresolver/strongly-connected-components/ ` `class` `Graph { ` `    ``int` `V; ` `    ``list<``int``>* adj; ` `    ``void` `fillOrder(``int` `v, ``bool` `visited[], ` `                      ``stack<``int``>& Stack); ` `    ``int` `DFSUtil(``int` `v, ``bool` `visited[]); ` ` `  `public``: ` `    ``Graph(``int` `V); ` `    ``void` `addEdge(``int` `v, ``int` `w); ` `    ``int` `countSCCNodes(); ` `    ``Graph getTranspose(); ` `}; ` ` `  `Graph::Graph(``int` `V) ` `{ ` `    ``this``->V = V; ` `    ``adj = ``new` `list<``int``>[V]; ` `} ` ` `  `// Counts number of nodes reachable ` `// from v ` `int` `Graph::DFSUtil(``int` `v, ``bool` `visited[]) ` `{ ` `    ``visited[v] = ``true``; ` `    ``int` `ans = 1; ` `    ``list<``int``>::iterator i; ` `    ``for` `(i = adj[v].begin(); i != adj[v].end(); ++i) ` `        ``if` `(!visited[*i]) ` `           ``ans += DFSUtil(*i, visited); ` `    ``return` `ans; ` `} ` ` `  `Graph Graph::getTranspose() ` `{ ` `    ``Graph g(V); ` `    ``for` `(``int` `v = 0; v < V; v++) { ` `        ``list<``int``>::iterator i; ` `        ``for` `(i = adj[v].begin(); i != adj[v].end(); ++i) ` `            ``g.adj[*i].push_back(v); ` `    ``} ` `    ``return` `g; ` `} ` ` `  `void` `Graph::addEdge(``int` `v, ``int` `w) ` `{ ` `    ``adj[v].push_back(w); ` `} ` ` `  `void` `Graph::fillOrder(``int` `v, ``bool` `visited[], ` `                           ``stack<``int``>& Stack) ` `{ ` `    ``visited[v] = ``true``; ` `    ``list<``int``>::iterator i; ` `    ``for` `(i = adj[v].begin(); i != adj[v].end(); ++i) ` `        ``if` `(!visited[*i]) ` `            ``fillOrder(*i, visited, Stack); ` `    ``Stack.push(v); ` `} ` ` `  `// This function mainly returns total count of  ` `// nodes in individual SCCs using Kosaraju's ` `// algorithm. ` `int` `Graph::countSCCNodes() ` `{ ` `    ``int` `res = 0; ` `    ``stack<``int``> Stack; ` `    ``bool``* visited = ``new` `bool``[V]; ` `    ``for` `(``int` `i = 0; i < V; i++) ` `        ``visited[i] = ``false``; ` `    ``for` `(``int` `i = 0; i < V; i++) ` `        ``if` `(visited[i] == ``false``) ` `            ``fillOrder(i, visited, Stack); ` `    ``Graph gr = getTranspose(); ` `    ``for` `(``int` `i = 0; i < V; i++) ` `        ``visited[i] = ``false``; ` `    ``while` `(Stack.empty() == ``false``) { ` `        ``int` `v = Stack.top(); ` `        ``Stack.pop(); ` `        ``if` `(visited[v] == ``false``) { ` `            ``int` `ans = gr.DFSUtil(v, visited); ` `            ``if` `(ans > 1) ` `                ``res += ans; ` `        ``} ` `    ``} ` `    ``return` `res; ` `} ` ` `  `// Returns count of cyclic elements in arr[] ` `int` `countCyclic(``int` `arr[], ``int` `n) ` `{ ` `    ``int`  `res = 0; ` ` `  `    ``// Create a graph of array elements ` `    ``Graph g(n + 1); ` ` `  `    ``for` `(``int` `i = 1; i <= n; i++) { ` `        ``int` `x = arr[i-1]; ` ` `  `        ``// If i + arr[i-1] jumps beyond last ` `        ``// element, we take mod considering ` `        ``// cyclic array ` `        ``int` `v = (x + i) % n + 1; ` ` `  `        ``// If there is a self loop, we ` `        ``// increment count of cyclic points. ` `        ``if` `(i == v) ` `            ``res++; ` ` `  `        ``g.addEdge(i, v); ` `    ``} ` ` `  `    ``// Add nodes of strongly connected components ` `    ``// of size more than 1. ` `    ``res += g.countSCCNodes(); ` ` `  `    ``return` `res; ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``int` `arr[] = {1, 1, 1, 1}; ` `    ``int` `n = ``sizeof``(arr)/``sizeof``(arr); ` `    ``cout << countCyclic(arr, n); ` `    ``return` `0; ` `} `

Output:

```4
```

Time Complexity : O(n)
Auxiliary space : O(n) Note that there are only O(n) edges.

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This article is attributed to GeeksforGeeks.org

## tags:

Arrays Graph graph-connectivity Arrays Graph

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