法一（自己思路，复杂了）：

from collections import deque

class Solution(object):
    def isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        queue = deque()
        if root.left!=None:
            queue.append(root.left)
            #节点的值可能是负数，所以要用列表
            queue.append([root.val])
            queue.append("0")
        if root.right!=None:
            queue.append(root.right)
            queue.append([root.val])
            queue.append("1")
        while len(queue)!=0:
            root = queue.popleft()
            # nodeVals = [].extend(queue.popleft())
            nodeVals = []
            nodeVals.extend(queue.popleft())
            paths = queue.popleft()
            for i in range(len(paths)):
                if paths[i]=="0" and root.val>=nodeVals[i]:
                    return False
                if paths[i]=="1" and root.val<=nodeVals[i]:
                    return False
            nodeVals.append(root.val)
            if root.left!=None:
                queue.append(root.left)
                queue.append(nodeVals)
                queue.append(paths+"0")
            
            if root.right!=None:
                queue.append(root.right)
                queue.append(nodeVals)
                queue.append(paths+"1")
        
        return True

法二（官方）：
利用深度遍历，如图，一看就懂

代码

class Solution {
    public boolean isValidBST(TreeNode root) {
        return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }

    public boolean isValidBST(TreeNode node, long lower, long upper) {
        if (node == null) {
            return true;
        }
        if (node.val <= lower || node.val >= upper) {
            return false;
        }
        return isValidBST(node.left, lower, node.val) && isValidBST(node.right, node.val, upper);
    }
}