Validate Binary Search Tree

Given a binary tree, determine if it is a valid binary search tree (BST).
Assume a BST is defined as follows:
The left subtree of a node contains only nodes with keys less than the node's key.
The right subtree of a node contains only nodes with keys greater than the node's key.
Both the left and right subtrees must also be binary search trees.

Solution: Recursion. 1. Add lower & upper bound. O(n)
2. Inorder traversal with one additional parameter (value of predecessor). O(n)

/**
 * 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) {
        int val = INT_MIN;
        return isValidBSTRe(root, val);
    }
    
    bool isValidBSTRe(TreeNode* root, int& val)
    {
        if(!root) return true;
        
        if(root->left && !isValidBSTRe(root->left, val))
            return false;
        if(root->val <= val) 
            return false;
        val = root->val;
        if(root->right && !isValidBSTRe(root->right, val))
            return false;
        return true;
    }
};