[LeetCode Sync] Runtime - 36 ms (96.38%), Memory - 19.6 MB (80.23%)

Author: NinePiece2

Solutions/0142-linked-list-cycle-ii/README.md

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+<p>Given the <code>head</code> of a linked list, return <em>the node where the cycle begins. If there is no cycle, return </em><code>null</code>.</p>
+
+<p>There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the <code>next</code> pointer. Internally, <code>pos</code> is used to denote the index of the node that tail&#39;s <code>next</code> pointer is connected to (<strong>0-indexed</strong>). It is <code>-1</code> if there is no cycle. <strong>Note that</strong> <code>pos</code> <strong>is not passed as a parameter</strong>.</p>
+
+<p><strong>Do not modify</strong> the linked list.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist.png" style="height: 145px; width: 450px;" />
+<pre>
+<strong>Input:</strong> head = [3,2,0,-4], pos = 1
+<strong>Output:</strong> tail connects to node index 1
+<strong>Explanation:</strong> There is a cycle in the linked list, where tail connects to the second node.
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test2.png" style="height: 105px; width: 201px;" />
+<pre>
+<strong>Input:</strong> head = [1,2], pos = 0
+<strong>Output:</strong> tail connects to node index 0
+<strong>Explanation:</strong> There is a cycle in the linked list, where tail connects to the first node.
+</pre>
+
+<p><strong class="example">Example 3:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test3.png" style="height: 65px; width: 65px;" />
+<pre>
+<strong>Input:</strong> head = [1], pos = -1
+<strong>Output:</strong> no cycle
+<strong>Explanation:</strong> There is no cycle in the linked list.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li>The number of the nodes in the list is in the range <code>[0, 10<sup>4</sup>]</code>.</li>
+	<li><code>-10<sup>5</sup> &lt;= Node.val &lt;= 10<sup>5</sup></code></li>
+	<li><code>pos</code> is <code>-1</code> or a <strong>valid index</strong> in the linked-list.</li>
+</ul>
+
+<p>&nbsp;</p>
+<p><strong>Follow up:</strong> Can you solve it using <code>O(1)</code> (i.e. constant) memory?</p>

Solutions/0142-linked-list-cycle-ii/solution.py

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+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+
+class Solution:
+    def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        fast = slow = head
+        while fast and fast.next:
+            slow = slow.next
+            fast = fast.next.next
+            if slow == fast:
+                result = head
+                while result != slow:
+                    result = result.next
+                    slow = slow.next
+                return result
+
+        # visited = set()
+        # current = head
+        # while current:
+        #     if current in visited:
+        #         return current
+        #     visited.add(current)
+        #     current = current.next
+        # return None