328. Odd Even Linked List

Example:
Given 1->2->3->4->5->NULL,
return 1->3->5->2->4->NULL.

//The program should run in O(1) space complexity and O(nodes) time complexity.

//Please note here we are talking about the node number and not the value in the nodes.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* oddEvenList(ListNode* head) {
        if (!head || !head->next ||!head->next->next)
            return head;

        ListNode * pWork = head;
        ListNode * end = head;
        int length = 1;
        while (end->next) {
            end = end->next;
            length++;
        }

        for (int i = 0; i < length/2; pWork = pWork->next,i++) {
            ListNode *tmp = pWork->next;
            pWork->next = tmp->next;
            end->next = tmp;
            tmp->next = NULL;
            end = tmp;
        }

        return head;
    }
};