98. Validate Binary Search Tree

public class Solution {
    public boolean isValidBST(TreeNode root) {
        Stack<TreeNode> stack=new Stack<TreeNode>();
        
        TreeNode cur=null;
        while(root!=null||!stack.isEmpty())
        {
            while(root!=null)
            {
                stack.push(root);
                root=root.left;
            }
            root=stack.pop();
            if(cur!=null&&cur.val>=root.val)
                return false;
            cur=root;
            root=root.right;
        }
        return true;
    }
}

class Solution {
    public boolean isValidBST(TreeNode root) {
        TreeNode cur=root, pre=null, p=null;
        while(cur!=null)
        {
            if(cur.left!=null)
            {
                p=cur.left;
                while(p.right!=null&&p.right!=cur)
                    p=p.right;
                if(p.right==null)
                {
                    p.right=cur;
                    cur=cur.left;
                }
                else
                {
                    if(pre!=null&&cur!=null&&pre.val>=cur.val)
                        return false;
                    pre=cur;
                    p.right=null;
                    cur=cur.right;
                }
            }
            else
            {
                if(pre!=null&&cur!=null&&pre.val>=cur.val)
                    return false;
                pre=cur;
                cur=cur.right;
            }
        }
        return true;
    }
}