LeetCode基础--二叉树--判断是否为BST

递归实现：

public class Solution {    public bool IsValidBST(TreeNode root) {        return isValidBST(root, long.MinValue, long.MaxValue);    }    private bool isValidBST(TreeNode root, long minVal, long maxVal)    {        if(root == null)        {            return true;        }        if(root.val >= maxVal || root.val <= minVal)        {            return false;        }        return isValidBST(root.left, minVal, root.val) && isValidBST(root.right, root.val, maxVal);    }}

非递归实现：

public class Solution {    public bool IsValidBST(TreeNode root) {        if(root == null)        {            return true;        }        Stack<TreeNode> stack = new Stack<TreeNode>();        TreeNode pre = null;        while(root != null || stack.Count != 0)        {            while(root != null)            {                stack.Push(root);                root = root.left;            }            root = stack.Pop();            if(pre != null && root.val <= pre.val)            {                return false;            }            pre = root;            root = root.right;        }        return true;    }}