solutions: 0876 - Middle of the Linked List (Easy)

Author: wingkwong

solutions/0800-0899/0876-middle-of-the-linked-list-easy.md

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+---
+description: 'Author: @wingkwong | https://leetcode.com/problems/middle-of-the-linked-list/description/'
+tags: [Linked List, Two Pointers]
+---
+
+# 0876 - Middle of the Linked List (Easy) 
+
+## Problem Link
+
+https://leetcode.com/problems/middle-of-the-linked-list/description/
+
+## Problem Statement
+
+Given the `head` of a singly linked list, return *the middle node of the linked list*.
+
+If there are two middle nodes, return **the second middle** node.
+
+**Example 1:**
+
+```
+Input: head = [1,2,3,4,5]
+Output: [3,4,5]
+Explanation: The middle node of the list is node 3.
+```
+
+**Example 2:**
+
+```
+Input: head = [1,2,3,4,5,6]
+Output: [4,5,6]
+Explanation: Since the list has two middle nodes with values 3 and 4, we return the second one.
+```
+
+**Constraints:**
+
+- The number of nodes in the list is in the range `[1, 100]`.
+- `1 <= Node.val <= 100`
+
+## Approach 1: Fast and Slow Pointer
+
+Classic Fast and Slow Pointer question
+
+- Time Complexity: $O(N)$ where $N$ is the number of nodes
+- Space Complexity: $O(1)$
+
+<Tabs>
+<TabItem value="cpp" label="C++">
+<SolutionAuthor name="@wingkwong"/>
+
+```cpp
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode() : val(0), next(nullptr) {}
+ *     ListNode(int x) : val(x), next(nullptr) {}
+ *     ListNode(int x, ListNode *next) : val(x), next(next) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode* middleNode(ListNode* head) {
+        // fast & slow pointer
+        // slow moves 1 step
+        // fast moves 2 steps
+        // 1 -> 2 -> 3 -> 4 -> 5
+        // slow : 1 -> 2 -> 3
+        // fast : 1 -> 3 -> 5
+        // when fast reaches the end, slow will be the middle
+        ListNode* slow = head;
+        ListNode* fast = head;
+        while (fast != NULL && fast->next != NULL) {
+            slow = slow->next;
+            fast = fast->next->next;
+        }
+        return slow;
+    }
+};
+```
+
+</TabItem>
+</Tabs>