解題思路：
morris遍歷

提交程式碼：

class Solution {
	public boolean isValidBST(TreeNode root) {
		TreeNode cur = root, pre=null,tmp=null;

		while (cur != null) {
			if (cur.left == null) {
				if(pre!=null&&pre.val>=cur.val)	return false;
				pre=cur;cur = cur.right;
				//if(pre.val>cur.val)	return false;
 

			} else {
				tmp = cur.left;
				while (tmp.right != null && tmp.right != cur)
					tmp = tmp.right;
				if (tmp.right == null) {
					tmp.right = cur;
					cur=cur.left;
				} else {
					if(pre.val>=cur.val)	return false;
					pre=cur;cur=cur.right;
					tmp.right = null;
				}
			}
 

		}
		return true;
	}
}

執行結果：