/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 *	TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定,请勿修改,直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return bool布尔型
     */
    void answer(TreeNode*root,vector <int> &ans)
    {
        if(root)
        {
            answer(root->left,ans);
            ans.push_back(root->val);
            answer(root->right,ans);
        }
    }
    bool isValidBST(TreeNode* root) 
    {
        vector <int> ans;
        answer(root,ans);
        vector <int> a=ans;
        sort(a.begin(),a.end());
        return a==ans;
    }
};