寻找链表中点 + 链表逆序 + 合并链表

/*** Definition for singly-linked list.* struct ListNode {*     int val;*     ListNode *next;*     ListNode() : val(0), next(nullptr) {}*     ListNode(int x) : val(x), next(nullptr) {}*     ListNode(int x, ListNode *next) : val(x), next(next) {}* };*/
class Solution {
public:void reorderList(ListNode* head) {ListNode* midnode = head;ListNode* fast  =head;while(fast != nullptr && fast->next != nullptr){midnode = midnode->next;fast = fast->next->next;}ListNode* left = head;ListNode* right = midnode->next;midnode->next  = nullptr;ListNode*pre = nullptr;ListNode*cur = right;while(cur != nullptr){ListNode* node = cur->next;cur->next = pre;pre = cur;cur = node;}right = pre;//ListNode* res = left;while(left->next != nullptr && right!=nullptr){ListNode* nextnode = left->next;left->next = right;right= right->next;left->next->next = nextnode;left=left->next->next;}// return res;}
};