In this problem while solvig getting the error use of undeclared identifer null
https://leetcode.com/problems/count-univalue-subtrees/solution/
Below is my solution any one have any idea why this is happening?

/**

• Definition for a binary tree node.

• struct TreeNode {

• int val;

• TreeNode *left;

• TreeNode *right;

• TreeNode() : val(0), left(nullptr), right(nullptr) {}

• TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}

• TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}

• };
  */
  #include <stddef.h>
  class Solution {
  public:
  #include <stddef.h>
  int count=0;
  bool isit(TreeNode node){
  if (node.left == null && node.right == null) {

       // found a univalue subtree - increment
       count++;
       return true;   
   }
  
   boolean is_unival = true;
  
   // check if all of the node's children are univalue subtrees and if they have the same value
   // also recursively call is_uni for children
   if (node.left != null) {
       is_unival = is_uni(node.left) && is_unival && node.left.val == node.val;
    }
   if (node.right != null) {
       is_unival = is_uni(node.right) && is_unival && node.right.val == node.val;
   }
  
   // return if a univalue tree exists here and increment if it does
   if (!is_unival) return false;
   count++;
   return true;
   
   
   

  }
  int countUnivalSubtrees(TreeNode* root) {
  if (root == null) return 0;
  is_uni(root);
  return count;

  }
  };