Leetcode: Count Univalue Subtrees

Given a binary tree, count the number of uni-value subtrees.

A Uni-value subtree means all nodes of the subtree have the same value.

For example:
Given binary tree,
              5
             / 
            1   5
           /    
          5   5   5
return 4.

The condition for a subtree to be a uni-value subtree is that:

(The left subtree has to be a uni-value subtree or null) && (right subtree has to be a uni-value subtree or null) && (left null || left.val== root.val) && (right null || right.val==root.val)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int countUnivalSubtrees(TreeNode root) {
        if (root == null) return 0;
        ArrayList<Integer> sum = new ArrayList<Integer>();
        sum.add(0);
        helper(root, sum);
        return sum.get(0);
    }
    
    public boolean helper(TreeNode root, ArrayList<Integer> sum) {
        if (root == null) return true;
        boolean left = helper(root.left, sum);
        boolean right = helper(root.right, sum);
        if (left && right && (root.left==null || root.val==root.left.val) && (root.right==null || root.val==root.right.val)) {
            sum.set(0, sum.get(0)+1);
            return true;
        }
        return false;
    }
}