Flatten Binary Tree to Linked List.txt

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 void flatten(TreeNode root){
        if(root == null) return;
        flat(root);
    }
    public TreeNode flat(TreeNode root){
        if(root == null) return null;
        TreeNode left = root.left;//先保存根节点的左右节点
        TreeNode right = root.right;
        root.left = null;//左节点为空右节点递归得到
        root.right = flat(left);
        TreeNode node = root;
        while(root.right != null)//循环至最右
            root = root.right;
        root.right = flat(right);
        return node;//返回根节点
    }
}