LeetCode Validate Binary Search Tree

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isValidBST(TreeNode *root) {
        return dfs(root, 0, 0, 0);
    }
    
    bool dfs(TreeNode *root, int lower, int upper, int part) {
        if (root == NULL) return true;
        if ((part & 0x1) && lower >= root->val) return false;
        if ((part & 0x2) && upper <= root->val) return false;
        return dfs(root->left, lower, root->val, part | 0x2) 
                && dfs(root->right, root->val, upper, part | 0x1);
    }
};