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/**
 * 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==NULL || head->next==NULL){
            return head;
        }
        ListNode * oddRunner=head;
        ListNode* evenHead=head->next;
        ListNode* evenRunner=evenHead;
        while(oddRunner->next!=NULL && evenRunner->next!=NULL){
            oddRunner->next=oddRunner->next->next; // set next as (jump two steps)
            evenRunner->next=evenRunner->next->next;
            oddRunner=oddRunner->next; // jump
            evenRunner=evenRunner->next;
        }
        oddRunner->next=evenHead;
        return head;
        
        
        // Method A: Using iterative two pointer
//         if(head==NULL || head->next==NULL){
//             return head;
//         }
//         ListNode* Odd=head;
//         ListNode* Evenhead=head->next;
//         ListNode* Even=Evenhead;
        
//         // starting from 3rd node
//         int i=3; // 1 indexed
//         ListNode* curr=NULL;
//         curr=Evenhead->next; // evenhead NULL is handeled in base case
//         while(curr!=NULL){
//             if(i&1){ // odd
//                 Odd->next=curr;
//                 Odd=Odd->next;
//             }else{
//                 Even->next=curr;
//                 Even=Even->next;
//             }
//             curr=curr->next;
//             i++;
//         }
        
//         if(Even!=NULL){
//             Even->next=NULL; // mark end at even of ll
//         }
//         Odd->next=Evenhead;
//         return head;
        
    }
};