leetcode 143 Reorder List

1. 

class Solution {
public:
    void reorderList(ListNode* head) {
        if(!head) return;
        vector<ListNode*> vec;
        ListNode* slow=head,*fast=head,*pre=nullptr;
        while(fast&&fast->next) {
            pre=slow;
            slow=slow->next;
            fast=fast->next->next;
        }
        if(fast) {
            pre=slow;
            slow=slow->next;
        }
        pre->next=nullptr;
        while(slow) {
            vec.push_back(slow);
            slow=slow->next;
        }
        slow=head;
        for(int i=vec.size()-1;i>=0&&slow;--i) {
            ListNode *node=vec[i],*tmp=slow->next;
            slow->next=node;node->next=tmp;
            slow=slow->next->next;
        }
    }
};

2. 

class Solution {
public:
    void reorderList(ListNode* head) {
        if(!head) return;
        ListNode* slow=head,*fast=head,*pre=nullptr;
        while(fast&&fast->next) {
            pre=slow;
            slow=slow->next;
            fast=fast->next->next;
        }
        if(fast) {
            pre=slow;
            slow=slow->next;
        }
        pre->next=nullptr;
        pre=nullptr;
        while(slow) {
            ListNode* next=slow->next;
            slow->next=pre;
            pre=slow;
            slow=next;
        }
        slow=head;
        while(slow&&pre) {
            ListNode* nextl=slow->next,*nextr=pre->next;
            slow->next=pre;
            pre->next=nextl;
            slow=nextl;
            pre=nextr;
        }
    }
};