0328.cpp

/**
 * 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) { return func1(head); }

  ListNode *func1(ListNode *head) {
    if (head == NULL || head->next == NULL) {
      return head;
    }

    ListNode *head1 = head;
    ListNode *curr1 = head1;
    ListNode *next1 = NULL;
    ListNode *tail1 = NULL;

    ListNode *head2 = head->next;
    ListNode *curr2 = head2;
    ListNode *next2 = NULL;

    while (curr1 != NULL && curr2 != NULL) {
      next1 = curr2->next;
      next2 = next1 == NULL ? NULL : next1->next;
      curr1->next = next1;
      curr2->next = next2;
      tail1 = curr1;
      curr1 = next1;
      curr2 = next2;
    }
    if (curr2 != NULL) {
      curr2->next = NULL;
    }
    if (curr1 == NULL) {
      tail1->next = head2;
    } else {
      curr1->next = head2;
    }

    return head1;
  }
};