114. 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
代码如下：

 /**
  * Definition for a binary tree node.
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 public class Solution {
     public void flatten(TreeNode root) {
         TreeNode p=root;
         List<TreeNode> list=PreOrder(root);

        for(int i=1;i<list.size();i++)
        {
            p.right=list.get(i);
            p.left=null;
            p=p.right;
        }

     }
     public List<TreeNode> PreOrder(TreeNode root){//树的前序遍历
          List<TreeNode> list=new ArrayList<>();
          try{
          list.add(root);

          if(root.left!=null)
          list.addAll(PreOrder(root.left));
          if(root.right!=null)
          list.addAll(PreOrder(root.right));
          }catch(NullPointerException e){}
          return list;
     }
 }