Flatten Binary Tree to Linked List

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
这道题，先先序遍历整棵树，在遍历的时候，构造一棵只有右子树的树。将root指向这颗新生成的树即可。

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    TreeNode newTree = null;
    
    public void flatten(TreeNode root) {
        if(null == root)
            return;
        preOrder(root);
        root.right = newTree.right;
        root.left = null;
    }
    
    /**
     *     前序遍历一棵树，生成一棵只有右子树的树
     * @param root
     * @return
     */
    public void preOrder(TreeNode root){
        if(null != root){
            if(newTree == null){                        //新树的根节点如果为空
                newTree = new TreeNode(root.val);
            }    
            else{                                        //根节点不为空
                TreeNode temp = newTree;
                
                while(temp.right != null)
                    temp = temp.right;                    //找到插入点
                temp.right = new TreeNode(root.val);    //在右子树插入结点                
            }
            preOrder(root.left);                        //遍历左子树
            preOrder(root.right);                        //遍历右子树
        }
    }
}