Flatten a binary tree to a fake "linked list" in pre-order traversal.
Here we use the right pointer in TreeNode as the next pointer in ListNode.
Don‘t forget to mark the left child of each node to null. Or you will get Time Limit Exceeded or Memory Limit Exceeded.
1
1 2
/ \ 2 5 => 3
/ \ \ 3 4 6 4
5
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[LintCode] Flatten Binary Tree to Linked List 将二叉树展开成链表
原文：http://www.cnblogs.com/grandyang/p/5642095.html