98 Validate Binary Search Tree

/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
bool isValidBST(TreeNode* root) {
if(root==nullptr)  return true;
vector<int> vec;
inOrderTraversal(root,vec);
for(int i=0;i<vec.size()-1;++i){
if(vec[i]>=vec[i+1])  return false;
}
return true;
}
void inOrderTraversal(TreeNode* root,vector<int> &vec){
if(root==nullptr)  return;
inOrderTraversal(root->left,vec);
vec.push_back(root->val);
inOrderTraversal(root->right,vec);
}
};