solves reorder list in java

Author: anishLearnsToCode

Diff for: README.md

@@ -127,7 +127,7 @@
 | 139  | [Word Break](https://leetcode.com/problems/word-break)                                                                                                    |                                                                    [![Java](assets/java.png)](src/WordBreak.java)                                                                    |                                                                                                                                   |
 | 141  | [Linked List Cycle](https://leetcode.com/problems/linked-list-cycle)                                                                                      |                                   [![Java](assets/java.png)](src/LinkedListCycle.java) [![Python](assets/python.png)](python/linked_list_cycle.py)                                   |                                                                                                                                   |
 | 142  | [Linked List Cycle II](https://leetcode.com/problems/linked-list-cycle-ii)                                                                                |                                                                [![Java](assets/java.png)](src/LinkedListCycleII.java)                                                                |                                                                                                                                   |
-| 143  | [Reorder List](https://leetcode.com/problems/reorder-list)                                                                                                |                                                                                                                                                                                      |                                                                                                                                   |
+| 143  | [Reorder List](https://leetcode.com/problems/reorder-list)                                                                                                |                                                                   [![Java](assets/java.png)](src/ReorderList.java)                                                                   |                                                                                                                                   |
 | 144  | [Binary Tree Preorder Traversal](https://leetcode.com/problems/binary-tree-preorder-traversal)                                                            |                      [![Java](assets/java.png)](src/BinaryTreePreOrderTraversal.java) [![Python](assets/python.png)](python/binary_tree_preorder_traversal.py)                       |                                                                                                                                   |
 | 145  | [Binary Tree Postorder Traversal](https://leetcode.com/problems/binary-tree-postorder-traversal)                                                          |                     [![Java](assets/java.png)](src/BinaryTreePostorderTraversal.java) [![Python](assets/python.png)](python/binary_tree_postorder_traversal.py)                      |                                                                                                                                   |
 | 146  | [LRU Cache](https://leetcode.com/problems/lru-cache)                                                                                                      |                                                                    [![Java](assets/java.png)](src/LRUCache.java)                                                                     |                                                                                                                                   |

Diff for: src/ReorderList.java

@@ -0,0 +1,47 @@
+// https://leetcode.com/problems/reorder-list
+// T: O(N)
+// S: O(1)
+
+public class ReorderList {
+    public void reorderList(ListNode head) {
+        if (head == null || head.next == null) return;
+
+        ListNode middle = getMiddleOfLinkedList(head);
+        ListNode preMiddle = reverseLinkedList(middle);
+        interleaveNodes(head, preMiddle);
+    }
+
+    private ListNode getMiddleOfLinkedList(ListNode head) {
+        ListNode p1 = head;
+        ListNode p2 = head;
+        while(p2.next!=null&&p2.next.next!=null){
+            p1 = p1.next;
+            p2 = p2.next.next;
+        }
+        return p1;
+    }
+
+    private ListNode reverseLinkedList(ListNode head) {
+        ListNode preCurrent = head.next;
+        while(preCurrent.next != null){
+            ListNode current = preCurrent.next;
+            preCurrent.next = current.next;
+            current.next = head.next;
+            head.next = current;
+        }
+        return head;
+    }
+
+    // 1->2->3->6->5->4 to 1->6->2->5->3->4
+    private void interleaveNodes(ListNode head, ListNode preMiddle) {
+        ListNode p1 = head;
+        ListNode p2 = preMiddle.next;
+        while(p1 != preMiddle){
+            preMiddle.next = p2.next;
+            p2.next = p1.next;
+            p1.next = p2;
+            p1 = p2.next;
+            p2 = preMiddle.next;
+        }
+    }
+}