//判断是否为BST 搜索树==二叉排序树 1、递归知最大最小值。2、先中序判是否单调

 bool IsValidBST(BTNode *p,int low,int high){

     if(p==NULL){

         return true;

     }else{

         if(low<p->data && high>p->data){

             return(IsValidBST(p->lchild,low,high) &&

                 IsValidBST(p->rchild,low,high));

         }else{

             return false;

         }

     }

 }

void IsBST(BTNode *p,int &k,bool &fail){

    if(p && !fail){

        IsBST(p->lchild,k,fail);

        if(k<p->data){

            k=p->data;

        }else{

            fail=true;

        }

        IsBST(p->rchild,k,fail);

    }

}

bool isValidBST(TreeNode *root) {

    vector<int> res;

    isValidBST(root, res);

    int len = res.size();

    bool flag = true;

    for (int i=; i<len-; i++){

        if (res[i] >= res[i+]){

            flag = false;

            break;

        }

    }

    return flag;

}

void isValidBSTOrder(TreeNode *root, vector<int> &res){

    if (root == NULL)

        return;

     isValidBST(root->left, res);

     res.push_back(root->val);

     isValidBST(root->right, res);

}

//判断是否为BST

bool fail=false;

void IsBST(BTNode *p,int &k,bool &fail){

    if(p && !fail){

        IsBST(p->lchild,k,fail);

        if(k<p->data){

            k=p->data;

        }else{

            fail=true;

        }

        IsBST(p->rchild,k,fail);

    }

}