98. 验证二叉搜索树

https://leetcode-cn.com/problems/validate-binary-search-tree/

//思路：中序遍历，升序
var preNode : TreeNode?


func isValidBST(_ root: TreeNode?) -> Bool {
    if root == nil {return true}
    
    if !isValidBST(root?.left) {return false}
    
    if (preNode != nil) && preNode!.val >= root!.val {return false}
    preNode = root
    
    if !isValidBST(root?.right) {return false}
    
    return true
}