构造二叉检索树本题目构造一棵二叉检索树。要求读入n个整数，以0结束。最后输出这棵树的先序序列。

以下是Python实现：

class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left = None
        self.right = None

def build_bst(nums):
    root = None
    for num in nums:
        if num == 0:
            break
        root = insert_node(root, num)
    return root

def insert_node(root, val):
    if root is None:
        return TreeNode(val)
    if val < root.val:
        root.left = insert_node(root.left, val)
    else:
        root.right = insert_node(root.right, val)
    return root

def preorder_traversal(root):
    if root is not None:
        print(root.val, end=" ")
        preorder_traversal(root.left)
        preorder_traversal(root.right)

if __name__ == '__main__':
    nums = []
    while True:
        num = int(input())
        if num == 0:
            break
        nums.append(num)
    root = build_bst(nums)
    preorder_traversal(root)

输入样例：

5
2
8
1
3
7
9
0

输出样例：

5 2 1 3 8 7 9