20. N-ary Tree Preorder Traversal.cpp

/*
N-ary Tree Preorder Traversal
=============================

Given the root of an n-ary tree, return the preorder traversal of its nodes' values.

Nary-Tree input serialization is represented in their level order traversal. Each group of children is separated by the null value (See examples)

Example 1:
Input: root = [1,null,3,2,4,null,5,6]
Output: [1,3,5,6,2,4]

Example 2:
Input: root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]
Output: [1,2,3,6,7,11,14,4,8,12,5,9,13,10]

Constraints:
The number of nodes in the tree is in the range [0, 104].
0 <= Node.val <= 104
The height of the n-ary tree is less than or equal to 1000.

Follow up: Recursive solution is trivial, could you do it iteratively?
*/

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

    Node() {}

    Node(int _val) {
        val = _val;
    }

    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/

class Solution
{
public:
  vector<int> preorder(Node *root)
  {
    if (!root)
      return {};
    vector<int> ans;
    ans.push_back(root->val);
    for (auto &child : root->children)
    {
      auto sub_ans = preorder(child);
      for (auto &i : sub_ans)
        ans.push_back(i);
    }
    return ans;
  }
};