class Solution {public:    vector
     
       V;    void postTree(TreeNode* node) {        if (node != NULL) {            if (node->left != NULL) {                postTree(node->left);            }            V.push_back(node->val);            if (node->right != NULL) {                postTree(node->right);            }        }    }    bool isValidBST(TreeNode* root) {        postTree(root);        for (int i = 1; i < V.size(); i++) {            if (V[i] <= V[i - 1]) {                return false;            }        }        return true;    }};