0589-n-ary-tree-preorder-traversal

589. N-ary Tree Preorder Traversal

Problem Description

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

Detail instruction can be found here.

Solution

Approach 1: Recursive

public List<Integer> preorder(Node root) {
    List<Integer> nodes = new ArrayList<>();
    traverse(root, nodes);
    return nodes;
}

private void traverse(Node x, List<Integer> nodes) {
    if (x == null)  return;
    nodes.add(x.val);
    for (Node descendent : x.children) {
        traverse(descendent, nodes);
    }
}

Approach 2: Iterative

public List<Integer> preorder(Node root) {        
    List<Integer> nodes = new ArrayList<>();
    if (root == null) return nodes;
    
    Stack<Node> stack = new Stack<>();
    stack.push(root);
    
    while (!stack.empty()) {
        Node x = stack.pop();            
        nodes.add(x.val);
        List<Node> children = x.children;
        for (int i = children.size() - 1; i >= 0; i--) {
            stack.push(children.get(i));
        }
    }
    
    return nodes;
}

** Definition for a Node**

class Node {
    public int val;
    public List<Node> children;

    public Node() {}

    public Node(int _val,List<Node> _children) {
        val = _val;
        children = _children;
    }
};

Test

Compile with javac Solution.java and run with java Solution.

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