对如下结构的树，执行travel函数，输出结果是1 2 3 4 5。

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

def travel(root):
    if not root:
        return
    stack = []
    stack.append(root)

    while stack:
        cur = stack.pop()
        print(cur.val, end=" ")
        if cur.right:
            stack.append(cur.right)
        if cur.left:
            stack.append(cur.left)

if __name__ == "__main__":
    root = Node(1)
    root.left = Node(2)
    root.right = Node(3)
    root.left.left = Node(4)
    root.left.right = Node(5)
    travel(root)