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0142-linked-list-cycle-ii/0142-linked-list-cycle-ii.java

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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) {
+        HashMap<ListNode, Integer> visited = new HashMap<>();
+        int i = 0;
+        while (head != null) {
+            if (!visited.containsKey(head)) {
+                visited.put(head, i);
+                i++;
+            }
+            else return head;
+            head = head.next;
+        }
+
+        return null;
+    }
+}