Flatten Binary Tree to Linked List

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.

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution  {    public TreeNode flatten(TreeNode root) {        if(root == null) {            return root;        }                TreeNode rightSubTree = root.right;        if(root.left != null) {            root.right = root.left;            root.left = null;            root = flatten(root.right);        }        if(rightSubTree != null) {            root.right = rightSubTree;            root = flatten(root.right);        }                return root;    }}