589. N-ary Tree Preorder Traversal

1. Description

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)

2. Example

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]

3. 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.

4. Follow Up

• Recursive solution is trivial, could you do it iteratively?

5. Solutions

Stack(Follow UP)

n is the number of nodes in root
Time complexity: O(n)
Space complexity: O(n)

class Solution {
public:
    vector<int> preorder(Node *root) {

        vector<int> preorder_values;
        stack<Node *> nodes;
        if (root != nullptr) {
            nodes.push(root);
        }

        while (!nodes.empty()) {
            auto node = nodes.top();
            nodes.pop();
            preorder_values.push_back(node->val);

            for (int i = node->children.size() - 1; i >= 0; --i) {
                nodes.push(node->children[i]);
            }
        }

        return preorder_values;
    }
};