algorithms/cpp/oddEvenLinkedList/OddEvenLinkedList.cpp

// Source : https://leetcode.com/problems/odd-even-linked-list/
// Author : Hao Chen
// Date   : 2016-01-16

/*************************************************************************************** 
 *
 * Given a singly linked list, group all odd nodes together followed by the even nodes. 
 * Please note here we are talking about the node number and not the value in the nodes.
 * 
 * You should try to do it in place. The program should run in O(1) space complexity 
 * and O(nodes) time complexity.
 * 
 * Example:
 * Given 1->2->3->4->5->NULL,
 * return 1->3->5->2->4->NULL.
 * 
 * Note:
 * The relative order inside both the even and odd groups should remain as it was in 
 * the input. 
 * The first node is considered odd, the second node even and so on ...
 * 
 * Credits:Special thanks to @aadarshjajodia for adding this problem and creating all 
 * test cases.
 ***************************************************************************************/

/**
 * 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 head;
        ListNode* pOdd = head;
        ListNode* p = head->next;
        ListNode* pNext = NULL;
        while(p && (pNext=p->next)) {
            
            p->next = pNext->next;
            pNext->next = pOdd->next;
            pOdd->next = pNext;
            
            p = p->next;
            pOdd = pOdd->next;
        
        }
        return head;
    }
};