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class Solution {
    public void flatten(TreeNode root) {
        if (root == null) return;
        
        Deque<TreeNode> stack = new ArrayDeque<>();
        stack.push(root);
        
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            // check right first because list is built at right leaf.
            if (node.right != null) {
                stack.push(node.right);
            }
            if (node.left != null) {
                stack.push(node.left);
            }
            if (!stack.isEmpty()) {
                node.right = stack.peek();
            }
            node.left = null;
        }
    }
}