589. N-ary Tree Preorder Traversal

DFS

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class Solution {
    public List<Integer> preorder(Node root) {
        // root -> child1 -> child2 -> child3

        List<Integer> resultList = new ArrayList<>();
        dfs(resultList, root);
        return resultList;
    }

    private void dfs(List<Integer> resultList, Node node) {
        if (node == null) {
            return;
        }

        resultList.add(node.val);

        if (node.children != null) {
            for (Node child : node.children) {
                dfs(resultList, child);
            }
        }
    }
}

Iterate

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class Solution {
    public List<Integer> preorder(Node root) {
        // root -> child1 -> child2 -> child3

        List<Integer> resultList = new ArrayList<>();

        Map<Node, Integer> nodeToNextChildIndexMap = new IdentityHashMap<>();

        Stack<Node> stack = new Stack<>();
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                resultList.add(root.val);
                stack.push(root);
                int childIndex = nodeToNextChildIndexMap.getOrDefault(root, 0);
                nodeToNextChildIndexMap.put(root, childIndex + 1);
                root = getChild(root, childIndex);
            }

            // now: root is null
            Node node = stack.pop(); // 节点已经添加过，需收集该节点的其他子节点
            int nextChildIndex = nodeToNextChildIndexMap.get(node);
            Node nextChild = getChild(node, nextChildIndex);
            if (nextChild != null) {
                root = nextChild;
                nodeToNextChildIndexMap.put(node, nextChildIndex + 1);
                stack.push(node); // node 的子节点还未遍历完成，继续将 node 入栈
            } else {
                nodeToNextChildIndexMap.remove(node);
            }
        }

        return resultList;
    }

    private Node getChild(Node node, int index) {
        if (node.children == null || index >= node.children.size()) {
            return null;
        }

        return node.children.get(index);
    }
}

Iterate

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class Solution {
    public List<Integer> preorder(Node root) {
        // root -> child1 -> child2 -> child3

        List<Integer> resultList = new ArrayList<>();

        Stack<Node> stack = new Stack<>();
        if (root != null) {
            stack.push(root);
        }

        while (!stack.isEmpty()) {
            Node node = stack.pop();
            resultList.add(node.val);

            if (node.children != null) {
                for (int i = node.children.size() - 1; i >= 0; i--) {
                    stack.push(node.children.get(i));
                }
            }
        }

        return resultList;
    }
}

References

589. N-ary Tree Preorder Traversal