Reorder List

Change-Id: I7d0abc3a6ac836a443ee7d0b280915f45625fce8

Author: applewjg

ReorderList.java

@@ -0,0 +1,81 @@
+/*
+ Author:     King, [email protected]
+ Date:       Jan 8, 2015
+ Problem:    Reorder List
+ Difficulty: Easy
+ Source:     http://oj.leetcode.com/problems/reorder-list/
+ Notes:
+ 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}.
+
+ Solution: Reverse the back half first, then reorder.
+*/
+
+
+/**
+ * Definition for singly-linked list.
+ * class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) {
+ *         val = x;
+ *         next = null;
+ *     }
+ * }
+ */
+public class Solution {
+    public void reorderList_1(ListNode head) {
+        ListNode slow = head, fast = head;
+        if (head == null || head.next == null) return;
+        while (fast.next != null && fast.next.next != null) {
+            fast = fast.next.next;
+            slow = slow.next;
+        }
+        fast = slow.next;
+        slow.next = null;
+        slow = head;
+        ListNode pre = null;
+        while (fast != null) {
+            ListNode next = fast.next;
+            fast.next = pre;
+            pre = fast;
+            fast = next;
+        }
+        fast = pre;
+        while (fast != null) {
+            ListNode fastnext = fast.next;
+            fast.next = slow.next;
+            slow.next = fast;
+            fast = fastnext;
+            slow = slow.next.next;
+        }
+    }
+    public void reorderList_2(ListNode head) {
+        ListNode slow = head, fast = head;
+        if (head == null || head.next == null) return;
+        while (fast.next != null && fast.next.next != null) {
+            fast = fast.next.next;
+            slow = slow.next;
+        }
+        fast = slow.next;
+        while (fast.next != null) {
+            ListNode move = fast.next;
+            fast.next = move.next;
+            move.next = slow.next;
+            slow.next = move;
+        }
+        fast = slow.next;
+        slow.next = null;
+        slow = head;
+        while (fast != null) {
+            ListNode fastnext = fast.next;
+            fast.next = slow.next;
+            slow.next = fast;
+            fast = fastnext;
+            slow = slow.next.next;
+        }
+    }
+}