Linked List Cycle II

LeetCode https://leetcode.cn/problems/linked-list-cycle-ii/

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {

    public ListNode detectCycle(ListNode head) {

        ListNode slow = head;
        ListNode fast = head;
        boolean hasCycle = false;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast) {
                hasCycle = true;
                break;
            }
        }
        
        // Proceed only if there is a cycle
        if (!hasCycle) {
            return null;
        }

        slow = head;
        while (slow != fast) {
            slow = slow.next;
            fast = fast.next;
        }

        return slow;
    }
}

Complexity

• Time = O(n)
• Space = O(1)

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