如何在C++中使用<<=运算符格式化链表

#include <iostream>

struct ListNode {
    int val;
    ListNode* next;
    ListNode(int x) : val(x), next(nullptr) {}
};

std::ostream& operator<<(std::ostream& os, const ListNode* head) {
    const ListNode* curr = head;
    while (curr != nullptr) {
        os << curr->val << " ";
        curr = curr->next;
    }
    return os;
}

int main() {
    ListNode* head = new ListNode(1);
    head->next = new ListNode(2);
    head->next->next = new ListNode(3);

    std::cout << head << std::endl;

    // 清理内存
    ListNode* curr = head;
    while (curr != nullptr) {
        ListNode* temp = curr;
        curr = curr->next;
        delete temp;
    }

    return 0;
}