328. Odd Even Linked List

/**
* 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) return NULL;
ListNode *p_odd = head;
ListNode *p_odd_prev = head;
ListNode *p_even_head = head->next;
if (!p_even_head) return head;
ListNode *p_even = p_even_head;

while (p_odd && p_even) {
p_odd->next = p_even->next;
p_odd_prev = p_odd;
p_odd = p_even->next;
if (p_odd) {
p_even->next = p_odd->next;
p_even = p_odd->next;
}
}

if (p_odd) {
p_odd->next = p_even_head;
} else {
p_odd_prev -> next = p_even_head;
}
return head;
}
};