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class ListNode:
    def __init__(self, val):
        self.val = val
        self.next = None

class Solution:
    # Function to detect start of loop using Hash Map
    def detectCycle(self, head):
        # Create a set to store visited nodes
        visited = set()

        # Traverse the list
        while head:
            # If current node already visited, it's the start of cycle
            if head in visited:
                return head

            # Add current node to visited set
            visited.add(head)

            # Move to next node
            head = head.next

        # No cycle found
        return None

# Driver code
if __name__ == "__main__":
    # Creating nodes
    head = ListNode(3)
    head.next = ListNode(2)
    head.next.next = ListNode(0)
    head.next.next.next = ListNode(-4)

    # Creating a cycle (tail connects to node index 1)
    head.next.next.next.next = head.next

    obj = Solution()
    startNode = obj.detectCycle(head)

    if startNode:
        print("Cycle starts at node with value:", startNode.val)
    else:
        print("No cycle found.")

12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849

class ListNode {
    constructor(val) {
        this.val = val;
        this.next = null;
    }
}

class Solution {
    // Function to detect start of loop using Set
    detectCycle(head) {
        // Create a set to store visited nodes
        let visited = new Set();

        // Traverse through the list
        while (head !== null) {
            // If node already in set, it's start of the loop
            if (visited.has(head)) {
                return head;
            }

            // Mark node as visited
            visited.add(head);

            // Move to next node
            head = head.next;
        }

        // If no cycle, return null
        return null;
    }
}

// Driver code
let head = new ListNode(3);
head.next = new ListNode(2);
head.next.next = new ListNode(0);
head.next.next.next = new ListNode(-4);

// Creating a cycle (tail connects to node index 1)
head.next.next.next.next = head.next;

let obj = new Solution();
let startNode = obj.detectCycle(head);

if (startNode)
    console.log("Cycle starts at node with value:", startNode.val);
else
    console.log("No cycle found.");