Recover binary search Tree - What's wrong with my code

My code can pass on my local environment, my fails the online judge. Anybody can help? thanks

public class Solution {
TreeNode n1 = null;
TreeNode n2 = null;
TreeNode prev = null;

private void inorder(TreeNode node){
    if(node == null) return;
    inorder(node.left);
    if(prev != null && prev.val > node.val) {
        if(n1 == null){
            n1 = prev;
        }
        n2 = node;
    } 
    prev = node;
    inorder(node.right);
}

public void recoverTree(TreeNode root) {
    // Start typing your Java solution below
    // DO NOT write main() function
    if(root == null) return;
    inorder(root);
    if(n1 != null && n2 != null){
        int swap = n1.val;
        n1.val = n2.val;
        n2.val = swap;
    }
}

}