class Solution {
    public TreeNode inorderSuccessor(TreeNode root, TreeNode p) {
       return inorderSuccessor(root, new TreeNode[]{p});
    }
    private TreeNode inorderSuccessor(TreeNode node, TreeNode[] p){
        if (node==null) return null;
        TreeNode left= inorderSuccessor(node.left, p);
        if (left!=null) return left;
        if (p[0]==null) return node;
        if (p[0]==node) p[0]= null;
        return inorderSuccessor(node.right, p);
    }
}