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

首先一个问题是怎么in-place flatten tree to a linkedlist?

应该给个提示就是 node.right ~~ node.next

ref http://www.cnblogs.com/springfor/p/3864355.html

public void flatten(TreeNode root) {
        if(root==null || (root.left==null && root.right==null)) return;
        Stack<TreeNode> st =  new Stack<TreeNode>();
        TreeNode n = root;
        while(n!=null || !st.isEmpty()){
            if(n.right!=null){
                st.push(n.right);
            }
            if(n.left!=null){
                n.right = n.left;
                n.left=null;
            }else if(!st.isEmpty()){
                TreeNode tmp=st.pop();
                n.right=tmp;
            }
            n=n.right;
        }
    }