给定一个二叉树，返回它的 前序 遍历。
示例:
输入: [1,null,2,3]
1
\
2
/
3
输出: [1,2,3]
代码
class Solution {
public:
vector<int> preorderTraversal(TreeNode* root) {
if (!root) return {};
vector<int> res;
stack<TreeNode*> s{{root}};
while (!s.empty()) {
TreeNode *t = s.top(); s.pop();
res.push_back(t->val);
if (t->right) s.push(t->right);
if (t->left) s.push(t->left);
}
return res;
}
};
