链表中，dummyhead是怎么做到自动更新到新的节点的呢？

比如leetcode206题，反转链表：

// Iterative
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if (!head) return head;
        ListNode *dummy = new ListNode(-1);
        dummy->next = head;
        ListNode *cur = head;
        while (cur->next) {
            ListNode *tmp = cur->next;
            cur->next = tmp->next;
            tmp->next = dummy->next;
            dummy->next = tmp;
        }
        return dummy->next;
    }
};

有明显的对dummy->next的操作，我可以理解。
但是对于leetcode86题：

class Solution {
public:
    ListNode *partition(ListNode *head, int x) {
        ListNode *dummy = new ListNode(-1);
        dummy->next = head;
        ListNode *pre = dummy, *cur = head;;
        while (pre->next && pre->next->val < x) pre = pre->next;
        cur = pre;
        while (cur->next) {
            if (cur->next->val < x) {
                ListNode *tmp = cur->next;
                cur->next = tmp->next;
                tmp->next = pre->next;
                pre->next = tmp;
                pre = pre->next;
            } else {
                cur = cur->next;
            }
        }
        return dummy->next;
    }
};

对于dummy->next的惟一一次操作在dummy->next = head;
我尝试输出了head以及pre，都不正确，那么dummy->next到底在什么时候更新的呢？

请问你想明白了吗？