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;
    }
};