590. N-ary Tree Postorder Traversal

DFS

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

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

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

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

        resultList.add(node.val);
    }
}

Iterate

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

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

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

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

            Node node = stack.pop();
            int nextChildIndex = nodeToNextChildIndexMap.get(node);
            Node nextChild = getChild(node, nextChildIndex);
            if (nextChild != null && nextChild != prev) {
                // 进入右子树，但不能重复进入
                root = nextChild;
                nodeToNextChildIndexMap.put(node, nextChildIndex + 1);
                stack.push(node); // node 节点为 root, 还需继续处理，所以需要入栈
            } else {
                // node is root
                resultList.add(node.val);
                prev = node;
            }
        }

        return resultList;
    }

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

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

References

590. N-ary Tree Postorder Traversal