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

tag: binary tree, DFS

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private class Chunk{
        TreeNode head;
        TreeNode tail;

        public Chunk(TreeNode head, TreeNode tail){
            this.head = head;
            this.tail = tail;
        }
    }
    public void flatten(TreeNode root) {
        if (root == null) return;
        helper(root);
    }

    private Chunk helper(TreeNode root){
        if (root == null) return null;

        Chunk leftChunk = helper(root.left);
        Chunk rightChunk = helper(root.right);
        root.left = null;
        Chunk curChunk = new Chunk(root, root);
        if (leftChunk != null) {
            curChunk.tail.right = leftChunk.head;
            curChunk.tail = leftChunk.tail;
        }
        if (rightChunk != null){
            curChunk.tail.right = rightChunk.head;
            curChunk.tail = rightChunk.tail;
        }


        return curChunk;
    }
}

一定要注意root.left = null;