文章作者：Tyan
博客：noahsnail.com  |  CSDN  |  简书

1. Description

N-ary Tree Preorder Traversal

2. Solution

• Recursive

/*
// Definition for a Node.
class Node {
public:
    int val;
    vector<Node*> children;

    Node() {}

    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/
class Solution {
public:
    vector<int> preorder(Node* root) {
        vector<int> result;
        if(!root) {
            return result;
        }
        preOrderTraverse(result, root);
        return result;
    }
    
    void preOrderTraverse(vector<int>& result, Node* root) {
        if(!root) {
            return;
        }
        result.push_back(root->val);
        int size = root->children.size();
        for(int i = 0; i < size; i++) {
            preOrderTraverse(result, root->children[i]);
        }
    }
};

• Iterative

/*
// Definition for a Node.
class Node {
public:
    int val;
    vector<Node*> children;

    Node() {}

    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/
class Solution {
public:
    vector<int> preorder(Node* root) {
        vector<int> result;
        if(!root) {
            return result;
        }
        preOrderTraverse(result, root);
        return result;
    }
    
    void preOrderTraverse(vector<int>& result, Node* root) {
        stack<Node*> list;
        list.push(root);
        while(!list.empty()) {
            Node* current = list.top();
            list.pop();
            result.push_back(current->val);
            int size = current->children.size();
            for(int i = size - 1; i >= 0; i--) {
                list.push(current->children[i]);
            }
        }
    }
};

Reference

1. https://leetcode.com/problems/n-ary-tree-preorder-traversal/description/