Create Reorder_List.java

Author: Anova07

LeetCode/Reorder_List.java

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+/**
+
+find the question here - https://leetcode.com/problems/reorder-list/
+
+
+ * 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; }
+ * }
+ 
+ 
+  Step 1: find the mid point
+   while(pt2.next!=null && pt2.next.next!=null){ 
+                pt1=pt1.next;
+                pt2=pt2.next.next
+  
+  Step 2: Reverse the end of the list
+  
+  Reverse the half after middle  1->2->3->4->5->6 to 1->2->3->6->5->4
+  
+  Step 3: merge both the list by reordering them one by one in order
+  
+   reorder one by one  1->2->3->6->5->4 to 1->6->2->5->3->4
+ */
+
+class Solution {
+    
+public void reorderList(ListNode head) {
+            if(head==null||head.next==null) return;
+            
+            //Find the middle of the list
+            ListNode pt1=head;
+            ListNode pt2=head;
+            while(pt2.next!=null && pt2.next.next!=null){ 
+                pt1=pt1.next;
+                pt2=pt2.next.next;
+            }
+            
+            //Reverse the half after middle  1->2->3->4->5->6 to 1->2->3->6->5->4
+            ListNode preMid=pt1;
+            ListNode preCurr=pt1.next;
+            while(preCurr.next!=null){
+                ListNode curr=preCurr.next;
+                preCurr.next=curr.next;
+                curr.next=preMid.next;
+                preMid.next=curr;
+            }
+            
+            //Start reorder one by one  1->2->3->6->5->4 to 1->6->2->5->3->4
+            pt1=head;
+            pt2=preMid.next;
+            while(pt1!=preMid){
+                preMid.next=pt2.next;
+                pt2.next=pt1.next;
+                pt1.next=pt2;
+                pt1=pt2.next;
+                pt2=preMid.next;
+            }
+        }
+}