Flatten Binary Tree to Linked List
Recursion:
public void flatten(TreeNode root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
    flattenRecurr(root);
    }
    private TreeNode flattenRecurr(TreeNode root) {
        if (root==null || root.left==null && root.right==null) return root;
TreeNode left = root.left;
TreeNode right = root.right;
root.left=null;

if (left!=null) {
root.right=left;
root=flattenRecurr(left);
}
if (right!=null) {
root.right=right;
root=flattenRecurr(right);
}
return root;
    }


Iterative:
 public void flatten(TreeNode root) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode p = root;
       
        while (p!=null || !stack.empty()) {
            if (p.right!=null) {
                stack.push(p.right);
            }
            if (p.left!=null) {
                p.right = p.left;
                p.left = null;
            } else if (!stack.empty()){
                TreeNode temp = stack.pop();
                p.right=temp;
            }
            p=p.right;
        }
    }

Same space complexity O(logn)