leetcode--二叉搜索树查询

给定一个二叉搜索树tree及特定值target，问target是否在tree中。
示例：
给定二叉树 tree = [3,9,20,null,null,15,7], target = 15
    3
   / \
  9  20
    /  \
   15   7

输出：True

def bin_tree_search(tree, target):
    if tree.value == None:
        return False
    
    while tree:
        if tree.value == target:
            return True
        elif tree.value > target:
            tree = tree.right
        else:
            tree = tree.left
    return False