250. Count Univalue Subtrees

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private int count = 0;
    public int countUnivalSubtrees(TreeNode root) {
        helper(root);
        return count;
    }
    
    private boolean helper(TreeNode root) {
        if (root == null) {
            return true;
        }
        boolean curUni = true;
        boolean left = helper(root.left);
        boolean right = helper(root.right);
        if (root.left != null) {
            curUni = curUni && left && (root.left.val == root.val);
        }
        if (root.right != null) {
            curUni = curUni && right && (root.right.val == root.val);
        }
         
        if (curUni) {
            count += 1;
        }
        return curUni;
    }
}