Create Reorder List.java

Author: Lokesh598

LinkedList/Reorder List.java

@@ -0,0 +1,48 @@
+/**
+ * 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 reverse(ListNode head) {
+        ListNode pre = null;
+        ListNode cur = null;
+
+        while (head!=null) {
+
+            pre = cur;
+            cur = head;
+            head = head.next;
+            cur.next = pre;
+        }
+        return cur;
+    }
+    public void reorderList(ListNode head) {
+        //1.find mid
+        ListNode slow = head;
+        ListNode fast = head;
+
+        while (fast!=null && fast.next!=null) {
+            slow = slow.next;
+            fast = fast.next.next;
+
+        }
+        //2. reverse half
+        ListNode s = reverse(slow.next);
+        slow.next = null;
+
+        while (head!=null && s!=null) {
+            ListNode nn = head.next;
+            ListNode rn = s.next;
+            head.next = s;
+            head.next.next = nn;
+            head=nn;
+            s=rn;
+        }
+    }
+}