对如下结构的树，执行 travel 函数，输出结果是 1 2 3 4 5 。
1 class Node: 2 def __init__(self, v): 3 self.val = v 4 self.left = None 5 self.right = None 6 7 def travel(root): 8 if not root: 9 return 10 stack = [] 11 stack.append(root) 12 13 while stack: = 14 cur = stack.pop() 15 print(cur.val, end=" ") 16 if cur.right: 17 stack.append(cur.right) 18 if cur.left: 19 stack.append(cur.left) 20 21 if __name__ == "__main__": 22 23 root = Node(1) 24 root.left = Node(2) 25 root.right = Node(3) 26 root.left.left = Node(4) 27 root.left.right = Node(5) 28 travel(root)
