250. Count Univalue Subtrees

public class Solution {
int cnt = 0;
public int countUnivalSubtrees(TreeNode root) {
helper(root);
return cnt;
}

private boolean helper(TreeNode root) {
if(root == null) {
return true;
}
boolean left = helper(root.left);
boolean right = helper(root.right);
if(left && right) {
if(root.left != null && root.val != root.left.val) {
return false;
}
if(root.right != null && root.val != root.right.val) {
return false;
}
cnt++;
return true;
}
return false;
}
}