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


解法：
iterative：

public void
 
 flatten(TreeNode root) {
        if (root==null) {
        return;
    }
   Stack<TreeNode> stack=new Stack<TreeNode>();
    stack.push(root);
   while(!stack.isEmpty()){
       TreeNode current=stack.pop();
       if (current.right!=null)  
           stack.push(current.right);
      if
 
 (current.left!=null)  
          stack.push(current.left);
      if (!stack.isEmpty()) 
          current.right = stack.peek();
      current.left=null;
      
   }
    }

recursive:

 public void flatten(TreeNode root) {
        if (root==null) {
        return;
    }
   TreeNode left=root.left;
   TreeNode right
 
=root.right;
   flatten(left);
   flatten(right);
   root.left=null;
   root.right=left;
   TreeNode tmp=root;
   while(tmp.right!=null)
   {
       tmp=tmp.right;
   }
   tmp.right=right;
    }

leetcode Flatten Binary Tree to Linked List