int helper(TreeNode* root, int& max) {
    if (root == NULL) {
      return 0;
    }

    int left = helper(root->left, max);
    int right = helper(root->right, max);    // step 1

    left = left < 0  0 : left;      // step 2
    right = right < 0  0 : right;
    max = max > (root->value + left + right)  max : (root->value + left + right);

    return left > right  (left + root->value) : (right + root->value);  // step 3
  }