589.n-ary-tree-preorder-traversa

递归

class Solution {
public:
    vector<int> preorder(Node* root) {
        if(root == nullptr) return {};
        vector<int> ans;
        preorder(root, ans);
        return ans;
    }
private:
    void preorder(Node* root, vector<int>& ans){
        if(root == nullptr) return;
        ans.push_back(root->val);
        for(auto n: root->children){
            preorder(n,ans);
        }
    }
};

迭代

用到了栈，反向迭代器

class Solution {
public:
    vector<int> preorder(Node* root) {
        // 方法：一个就是队列,一个就是栈   
        if(!root) return {};
        stack<Node*> s;
        s.push(root);
        vector<int> ans;
        while(!s.empty()){
            Node* curNode = s.top();s.pop();
            ans.push_back(curNode->val);   
            for(auto n_iter= curNode->children.rbegin();n_iter != curNode->children.rend(); n_iter++){
                s.push(*n_iter);
            }
        }
        return ans; 
    }
};