leetcode Flatten Binary Tree to Linked List

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;
      
   }
    }

 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;
    }