对如下结构的树，执行travel函数，输出结果是 1 2 3 4 5。

1      12     / \3     2 34    / \5   4  5

1struct Node {2   int val;3   Node *left, *right;4   Node(int v) : val(v), left(nullptr), right(nullptr) {}5};6 7void travel(Node* root) {8   if (!root) return;9   stack<Node*> s;10   s.push(root);11 12   while (!s.empty()) {13      Node* cur = s.top(); s.pop();14      cout << cur->val << " ";15 16      if (cur->right) s.push(cur->right);17      if (cur->left) s.push(cur->left);18   }19}