Longest Cycle in a Graph LeetCode Solution

Problem – Longest Cycle in a Graph LeetCode Solution

You are given a directed graph of n nodes numbered from 0 to n - 1, where each node has at most one outgoing edge.

The graph is represented with a given 0-indexed array edges of size n, indicating that there is a directed edge from node i to node edges[i]. If there is no outgoing edge from node i, then edges[i] == -1.

Return the length of the longest cycle in the graph. If no cycle exists, return -1.

A cycle is a path that starts and ends at the same node.

Example 1:

Input: edges = [3,3,4,2,3]
Output: 3
Explanation: The longest cycle in the graph is the cycle: 2 -> 4 -> 3 -> 2.
The length of this cycle is 3, so 3 is returned.

Example 2:

Input: edges = [2,-1,3,1]
Output: -1
Explanation: There are no cycles in this graph.

Constraints:

  • n == edges.length
  • 2 <= n <= 105
  • -1 <= edges[i] < n
  • edges[i] != i

Longest Cycle in a Graph LeetCode Solution in C++

   int maxLength = -1;

    void getcycle(vector<int> &edges,int si,vector<bool>& visit,int size,vector<int>& store){
        if(si == -1)return ;
        if(visit[si]){
            int count = -1;
            for(int i =0;i<store.size();i++){
                if(store[i]==si){
                    count = i;
                    break;
                } }
            
            if(count==-1)return;
            maxLength = max(maxLength,size-count);  // size-count gives the proper size of cycle
            return ;
        }
        
        visit[si] = true;
        store.push_back(si);
        getcycle(edges,edges[si],visit,size+1,store);
        
        return ;
        
    }
    
    
    
    int longestCycle(vector<int>& edges) {
        
        vector<bool> visit(edges.size(),0);
        for(int i =0;i<edges.size();i++){
            
            if(visit[i])continue;
            vector<int> store;
            getcycle(edges,i,visit,0,store);
     
        }
        
        return maxLength;
        
    }

Longest Cycle in a Graph LeetCode Solution in Java

public int longestCycle(int[] edges) {
    int res = -1;
    boolean[] vis = new boolean[edges.length]; // global visisted
    
    for(int i=0; i<edges.length; i++){
        if(vis[i]) continue;
        int dist = 0, idx = i;
        HashMap<Integer, Integer> x = new HashMap<>();  // local visited
        while(idx!=-1){
            if(x.containsKey(idx)){
                res = Math.max(res, dist-x.get(idx));
                break;
            }
            if(vis[idx]) break;
            vis[idx] = true;
            x.put(idx, dist++);
            idx = edges[idx];
        } 
    }
    return res;
}

Longest Cycle in a Graph LeetCode Solution in Python

def longestCycle(self, edges: List[int]) -> int:
    n = len(edges)
    self.max_length = float('-inf')
    seen = [False] * n
    visiting = {}
    stack = []
        
    def dfs(node):
        if not seen[node]:
            if node in visiting:
                self.max_length = max(self.max_length, len(stack) - visiting[node])
            elif edges[node] != -1: 
                visiting[node] = len(stack) 
                stack.append(node)
                dfs(edges[node])
                stack.pop()
                visiting.pop(node)
            seen[node] = True
  
    for i in range(n):            
        dfs(i)
    return self.max_length if self.max_length > 0 else -1    
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