feat: 2025-10-23打卡

142.linked-list-cycle-ii.java

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+/*
+ * @lc app=leetcode id=142 lang=java
+ *
+ * [142] Linked List Cycle II
+ */
+
+// @lc code=start
+/**
+ * Definition for singly-linked list.
+ * class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) {
+ *         val = x;
+ *         next = null;
+ *     }
+ * }
+ */
+public class Solution {
+    /**
+     * 2 * k - s - (k - s) = p * r
+     * 2 * k - s - k + s = p * r
+     * k = p * r, 且 k > s
+     * 
+     * s + p * r + m - 2m = q * r
+     * s = q * r - p * r  + m
+     * 
+     * @param head
+     * @return
+     */
+    public ListNode detectCycle(ListNode head) {
+        if(head == null) {
+            return null;
+        }
+        Set<ListNode> tracked = new HashSet<>();
+        ListNode cur = head, r = null;
+        while(cur != null) {
+            if(tracked.contains(cur)) {
+                r = cur;
+                break;
+            }
+            tracked.add(cur);
+            cur = cur.next;
+        }
+        return r;
+    }
+}
+// @lc code=end
+