Reorder List

Author: fishercoder1534

README.md

@@ -161,6 +161,7 @@ Your ideas/fixes/algorithms are more than welcome!
 |149|[Max Points on a Line](https://leetcode.com/problems/max-points-on-a-line/)|[Solution](../../blob/master/src/stevesun/algorithms/MaxPointsonaLine.java)| O(?)|O(?) | Hard|
 |145|[Binary Tree Postorder Traversal](https://leetcode.com/problems/binary-tree-postorder-traversal/)|[Solution](../../blob/master/src/stevesun/algorithms/BinaryTreePostOrderTraversal.java)| O(n)|O(h) | Hard| Binary Tree
 |144|[Binary Tree Preorder Traversal](https://leetcode.com/problems/binary-tree-preorder-traversal/)|[Solution](../../blob/master/src/stevesun/algorithms/BinaryTreePreorderTraversal.java)| O(n)|O(h) | Medium| Binary Tree
+|143|[Reorder List](https://leetcode.com/problems/reorder-list/)|[Solution](../../blob/master/src/stevesun/algorithms/ReorderList.java)| O(n)|O(1) | Medium|
 |142|[Linked List Cycle II](https://leetcode.com/problems/linked-list-cycle-ii/)|[Solution](../../blob/master/src/stevesun/algorithms/LinkedListCycleII.java)| O(n)|O(1) | Medium| Linked List
 |141|[Linked List Cycle](https://leetcode.com/problems/linked-list-cycle/)|[Solution](../../blob/master/src/stevesun/algorithms/LinkedListCycle.java)| O(n)|O(1) | Easy| Linked List
 |140|[Word Break II](https://leetcode.com/problems/word-break-ii/)|[Solution](../../blob/master/src/stevesun/algorithms/WordBreakII.java)| ? |O(n^2) | Hard| Backtracking/DFS

src/stevesun/algorithms/ReorderList.java

@@ -0,0 +1,59 @@
+package stevesun.algorithms;
+
+import stevesun.common.classes.ListNode;
+
+/**
+ * Given a singly linked list L: L0→L1→…→Ln-1→Ln,
+ reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…
+
+ You must do this in-place without altering the nodes' values.
+
+ For example,
+ Given {1,2,3,4}, reorder it to {1,4,2,3}.
+ */
+public class ReorderList {
+
+    public void reorderList(ListNode head) {
+        if (head == null || head.next == null) {
+            return;
+        }
+		/* first we use two pointers to separate this list into two parts */
+        ListNode slowNode = head, fastNode = head;
+        while (fastNode.next != null) {
+            fastNode = fastNode.next;
+            if (fastNode.next != null) {
+                fastNode = fastNode.next;
+            } else {
+                break;
+            }
+            slowNode = slowNode.next;
+        }
+        // two sublist heads
+        ListNode head1 = head, head2 = slowNode.next;
+        // detach the two sublists;
+        slowNode.next = null;
+
+        // reverse the second sublist
+        ListNode cur = head2, post = cur.next;
+        cur.next = null;
+        while (post != null) {
+            ListNode temp = post.next;
+            post.next = cur;
+            cur = post;
+            post = temp;
+        }
+        head2 = cur;// the new head of the reversed sublist
+
+        // merge the two sublists as required
+        ListNode p = head1, q = head2;
+        while (q != null) {
+            ListNode temp1 = p.next;
+            ListNode temp2 = q.next;
+            p.next = q;
+            q.next = temp1;
+            p = temp1;
+            q = temp2;
+        }
+    }
+
+}