[Binary tree algorithm] to determine whether the algorithm on the BST

// determines whether BST search tree binary sort tree == 1, recursive known maximum and minimum. 2, in order to judge whether or not monotonic 
 BOOL IsValidBST (BTNode * P, int Low, int High) {
      IF (P == NULL) {
          return  to true ; 
     } the else {
          IF (Low <p-> High Data &&> p-> Data) {
              return (IsValidBST (p-> lchild, Low, High) && 
                 IsValidBST (P -> rchild, Low, High)); 
         } the else {
              return  to 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=0; i<len-1; i++){
        if (res[i] >= res[i+1]){
            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);
    }
}