题目：

题解：

class Solution {
public:void reorderList(ListNode* head) {if (head == nullptr) {return;}ListNode* mid = middleNode(head);ListNode* l1 = head;ListNode* l2 = mid->next;mid->next = nullptr;l2 = reverseList(l2);mergeList(l1, l2);}ListNode* middleNode(ListNode* head) {ListNode* slow = head;ListNode* fast = head;while (fast->next != nullptr && fast->next->next != nullptr) {slow = slow->next;fast = fast->next->next;}return slow;}ListNode* reverseList(ListNode* head) {ListNode* prev = nullptr;ListNode* curr = head;while (curr != nullptr) {ListNode* nextTemp = curr->next;curr->next = prev;prev = curr;curr = nextTemp;}return prev;}void mergeList(ListNode* l1, ListNode* l2) {ListNode* l1_tmp;ListNode* l2_tmp;while (l1 != nullptr && l2 != nullptr) {l1_tmp = l1->next;l2_tmp = l2->next;l1->next = l2;l1 = l1_tmp;l2->next = l1;l2 = l2_tmp;}}
};