LeetCode: Reorder List [143]

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reserve(ListNode *head){
        if(head==NULL || head->next==NULL)return head;
        ListNode* prev=NULL;
        ListNode* cur=head;
        ListNode* next=NULL;
        while(cur){
            next=cur->next;
            cur->next=prev;
            prev=cur;
            cur=next;
        }
        return prev;
    }
    ListNode* findMid(ListNode *head){
        if(head==NULL || head->next==NULL)return head;
        ListNode*prev=NULL;
        ListNode*p1=head;
        ListNode*p2=head;
        while(p2){
            prev=p1;
            p1=p1->next;
            p2=p2->next;
            if(p2)p2=p2->next;
        }
        prev->next=NULL;
        return p1;
    }
    ListNode* merge(ListNode*head1, ListNode*head2){
        ListNode*p1=head1;
        ListNode*p2=head2;
        ListNode*head=NULL, *p=NULL;
        while(p2){
            if(head==NULL)head=p1; 
            else p->next=p1;
            p=p1;
            p1=p1->next;
            p->next=p2;
            p=p2;
            p2=p2->next;
        }
        p->next=p1;
        return head;
    }
    void reorderList(ListNode *head) {
        if(head==NULL || head->next==NULL)return;
        ListNode* head2 = findMid(head);
        head2 = reserve(head2);
        head=merge(head, head2);
    }
};