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Copy pathLinkedListCycleII.java
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70 lines (60 loc) · 1.56 KB
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package com.algorithm.playground.leetcode.problems.lc100.lc140.lc142;
import com.algorithm.playground.leetcode.problems.utils.linkedlist.ListNode;
import java.util.IdentityHashMap;
import java.util.Map;
/**
* https://leetcode.com/problems/linked-list-cycle-ii/
* <p>
* Given a linked list, return the node where the cycle begins. If there is no cycle, return null.
* <p>
* To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed)
* in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.
* <p>
* Note: Do not modify the linked list.
*/
public class LinkedListCycleII {
/**
* 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, fast = head;
while (fast != null && fast.next != null) {
slow = slow.next;
fast = fast.next.next;
if (slow == fast) {
break;
}
}
if (fast == null || fast.next == null) {
return null;
}
slow = head;
while (slow != fast) {
slow = slow.next;
fast = fast.next;
}
return slow;
}
}
public class SolutionWithAdditionalMemory {
public ListNode detectCycle(ListNode head) {
Map<ListNode, ListNode> seen = new IdentityHashMap<>();
for (; head != null; head = head.next) {
if (seen.containsKey(head)) {
return head;
}
seen.put(head, head);
}
return null;
}
}
}