docs(docs): add documentation for LeetCode 141 Linked List Cycle

apps/docs/content/(hash-table)/141-linked-list-cycle.mdx

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+---
+title: '141. Linked List Cycle'
+description: Given head, the head of a linked list, determine if the linked list has a cycle in it
+sidebar:
+  label: 'Linked List Cycle'
+  badge: 'Easy'
+---
+
+<Badge variant="accent">Linked List</Badge>
+
+::::warning
+Can you solve it using O(1) (i.e. constant) memory?
+::::
+
+### Example 1:
+- Input: `head = [3,2,0,-4], pos = 1`
+- Output: `true`
+- Explanation: There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed).
+
+### Example 2:
+- Input: `head = [1,2], pos = 0`
+- Output: `true`
+- Explanation: There is a cycle in the linked list, where the tail connects to the 0th node.
+
+### Example 3:
+- Input: `head = [1], pos = -1`
+- Output: `false`
+- Explanation: There is no cycle in the linked list.
+
+### Constraints:
+
+- The number of the nodes in the list is in the range [0, 10^4].
+- `-10^5 <= Node.val <= 10^5`
+- pos is -1 or a valid index in the linked-list.
+
+## Solution
+
+```py
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+
+
+class Solution:
+    def hasCycle(self, head: Optional[ListNode]) -> bool:
+        fast = slow = head
+
+        while fast and fast.next:
+            fast = fast.next.next
+            slow = slow.next
+            if fast is slow:
+                return True
+        return False
+```