N叉树前中后序遍历模板

// 递归，前序遍历
public void preorder(Node root) {
        if (root == null) {
            return;
        }

        /*用root在这里写业务代码*/

        for (Node node : root.children) {
            preorder(node, result);
        }
    }

// 迭代，前序遍历
public void preorder(Node root) {
        Stack<Node> stack = new Stack<>();
        if (root == null) {
            return;
        }

        stack.push(root);
        while (!stack.isEmpty()) {
            Node pop = stack.pop();

            /*用root在这里写业务代码*/

            for (int i = pop.children.size() - 1; i >= 0; i --) { // 注意这里是从大到小压栈
                stack.push(pop.children.get(i));
            }
        }
    }

// 递归，后序遍历
public void postorder(Node root) {
        if (root == null) {
            return;
        }

        for (Node node : root.children) {
            helper(node, result);
        }
        
        /*用root在这里写业务代码*/

    }


// 迭代，后序遍历
public void postorder(Node root) {
        List<Integer> result = new ArrayList<>();
        Set<Node> visited = new HashSet<>();
        Stack<Node> stack = new Stack<>();
        if (root == null) {
            return result;
        }
        stack.push(root);
        while (!stack.isEmpty()) {
            Node peek = stack.peek();
            if (peek.children.size() == 0 || visited.contains(peek)) {
                Node pop = stack.pop();
                
                /*用pop在这里写业务代码*/

                continue;
            }

            for (int i = peek.children.size() - 1; i >= 0; i --) { // 注意这里从大到小压栈
                stack.push(peek.children.get(i));
            }
            visited.add(peek);
        }
    }