0589 - N-ary Tree Preorder Traversal (Easy)

Problem Link

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

Problem Statement

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, 10^4].
• 0 <= Node.val <= 10^4
• The height of the n-ary tree is less than or equal to 1000.

Approach 1: DFS

Straightforward preorder traversal.

class Solution {
public:
    vector<int> ans;
    void dfs(Node* node) {
        if (!node) return;
        ans.push_back(node->val);
        for (auto x : node->children) dfs(x);
    }
    vector<int> preorder(Node* root) {
        dfs(root);
        return ans;
    }
};