reorder-list.py

Author: Chris Wu

problems/reorder-list.py

@@ -0,0 +1,39 @@
+"""
+The reorder list, we can see it as
+smallest, largest, 2th smallest, 2th largest, 3th smallest...
+
+First, put the ordered nodes in a stack, so we can get the largest node by `pop()`.
+And count how many nodes in total.
+
+Second, because the node is already ordered
+So we only need to insert a large node between every node until we hit the count.
+
+Last, remember to put `None` at the end.
+
+The tricky part is there may be even count or odd count of nodes in total.
+When face problem like this, you should approach these test cases by hand.
+Think about the edge cases. Here:
+1->2->3->4
+1->2->3->4->5
+"""
+class Solution(object):
+    def reorderList(self, head):
+        if head is None: return
+        h = head
+        curr = head
+        stack = []
+        count = 0
+
+        while curr:
+            count+=1
+            stack.append(curr)
+            curr = curr.next
+
+        while count>1:
+            temp = h.next
+            h.next = stack.pop()
+            h.next.next = temp
+            count-=2
+            h = h.next.next
+
+        h.next = None