LC.P142[环形链表II]

方法一：哈希表

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/**
 * 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) {
        Set<ListNode> set = new HashSet<>();
        ListNode p = head;
        while (p != null) {
            if (!set.add(p)) return p;
            p = p.next;
        }
        return null;
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$

方法二：快慢指针

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/**
 * 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 fast = head, slow = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            // 存在环
            if (slow == fast) {
                ListNode ans = head;
                while (ans != slow) {
                    ans = ans.next;
                    slow = slow.next;
                }
                return ans;
            }
        }
        return null;
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(1)$