BST properties (Tree DFS Sub-pattern)

// BST: every node must fit inside the (low, high) range set by its ancestors.
// long long bounds so a node equal to INT_MIN or INT_MAX still fits.
bool valid(TreeNode* node, long long low, long long high) {
    if (!node) return true;
    if (node->val <= low || node->val >= high) return false;
    return valid(node->left, low, node->val)             // left side: everything below node->val
        && valid(node->right, node->val, high);          // right side: everything above node->val
}

bool isValidBST(TreeNode* root) { return valid(root, LLONG_MIN, LLONG_MAX); }

// Searching only needs one side: the comparison tells you where the value must be.
TreeNode* searchBST(TreeNode* root, int target) {
    while (root && root->val != target)
        root = target < root->val ? root->left : root->right;
    return root;
}