Linked List: Find Middle of the Linked List in Java #361

Author: alaka03aj

Linked List/MiddleOfLinkedList.java

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+/* QUESTION and SAMPLE I/O
+
+    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.
+
+    Sample Input = [1,2,3,4,5]
+    Sample Output = 3
+    
+   APPROACH and EXPLANATION
+   
+    This program is done using two pointers: slow pointer and fast pointer. 
+        * Both pointer starts from the head node.
+        * Slow pointer moves one step at a time.
+        * Fast pointer moves two steps at a time.
+        * As the fast pointer reaches the end, slow pointer will be at the middle of the linked list.
+        * Accessing the slow pointer's value for 'val' will give you the value for the middle element of linked list.
+
+
+    Time Complexity: O(n)
+    Space Complexity: O(1)
+
+ */
+
+ 
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode() {}
+ *     ListNode(int val) { this.val = val; }
+ *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+ * }
+ */
+
+class Solution {
+    public ListNode middleNode(ListNode head) {
+        ListNode slow_pointer = head;       //initialize slow pointer
+        ListNode fast_pointer = head;       //initialize fast pointer
+        while(fast_pointer!=null && fast_pointer.next!=null){
+            slow_pointer = slow_pointer.next;       //moves to next node
+            fast_pointer = fast_pointer.next.next;      //moves two nodes forward
+        }
+        return slow_pointer;
+    }
+}