Flatten Binary Tree to Linked List ---LeetCode

/**
* 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) {
if (root == null) return ;
Stack<TreeNode> stack = new Stack<>();

while (root != null || !stack.isEmpty()) {
if (root.right != null) {
stack.push(root.right);
}

if (root.left != null) {
root.right = root.left;
root.left = null;
} else if (!stack.isEmpty()) {
root.right = stack.pop();
}
root = root.right;
}
}
}

private TreeNode prev = null;

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