leetcode

Author: shangchun

reorder.py

@@ -0,0 +1,70 @@
+# Definition for singly-linked list.
+class ListNode:
+    def __init__(self, x):
+        self.val = x
+        self.next = None
+
+# @param head, a ListNode
+# @return nothing
+def reorderList(head):
+    # count
+    count = 0
+    node = head
+    tail = node
+    while node:
+        count += 1
+        tail = node
+        node = node.next
+    if count <= 2:
+        return
+
+    # reverse right half
+    step = 0
+    pivot = (count - 1) / 2;
+    behind = head
+    ahead = behind.next
+    while ahead:
+        if step < pivot:
+            behind = ahead
+            ahead = ahead.next
+        else:
+            node = ahead.next
+            if step == pivot:
+                behind.next = None
+                ahead.next = None
+            else:
+                ahead.next = behind
+            behind = ahead
+            ahead = node
+        step += 1
+
+    # insert
+    while head and tail:
+        nodeHead = head.next
+        nodeTail = tail.next
+        head.next = tail
+        tail.next = nodeHead
+        head = nodeHead
+        tail = nodeTail
+
+a = ListNode('a')
+b = ListNode('b')
+c = ListNode('c')
+d = ListNode('d')
+a.next = b
+b.next = c
+c.next = d
+reorderList(a)
+assert a.next == d
+assert b.next == c
+assert c.next == None
+
+e = ListNode('e')
+f = ListNode('f')
+g = ListNode('g')
+e.next = f
+f.next = g
+reorderList(e)
+assert e.next == g
+assert g.next == f
+assert f.next == None