114. Flatten Binary Tree to Linked List

题目

Given a binary tree, flatten it to a linked list in-place.

For example,
Given
         1
        / \
       2   5
      / \   \
     3   4   6

The flattened tree should look like:
   1
    \
     2
      \
       3
        \
         4
          \
           5
            \
             6

click to show hints.

Hints: If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.

解题思路

见程序注释