Validate Binary Search Tree(cpp, leetcode,tree)

 Given a binary tree, determine if it is a valid binary search tree (BST).

Example 2:

    5
   / \
  1   4
     / \
    3   6


Output: false
Explanation: The root node's value is 5 but its right child's value is 4. 

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class Solution {
public:
    bool isValidBST(TreeNode* root) 
    {
        stack<TreeNode*>s;
        
        vector<int>a;
        if(root==NULL)
            return true;
        
        s.push(root);
        root=root->left;
        while(1)
        {
            while(root)
            {
                s.push(root);
                root=root->left;
            }
            if(s.empty())
            {
                break;
            }
            root=s.top();
            s.pop();
            a.push_back(root->val);
            root=root->right;
            
            
        }
        
        for(int i=0;i<a.size()-1;i++)
        {
            if(a[i]>=a[i+1])
                return false;
        }
        return true;
    }
};