leetcode - Reorder List

 

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    void reorderList(ListNode *head) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if (head ==NULL || head->next == NULL){
            return;
        }
        // split the lists to two parts
        ListNode * lhead=head, *ltail=head, *rhead=head->next; // split the lists to two parts
        ListNode * itr = head->next;
        while ((itr!=NULL) && itr->next !=NULL){
            ltail = ltail->next;
            rhead = rhead->next;
            itr = (itr->next)->next;
        }
        ltail->next = NULL;
        //reverse the right part
        ListNode * last = rhead;
        itr = rhead->next;
        while (itr!=NULL){
            ListNode * tmp = itr->next;
            itr->next = last;
            last = itr;
            itr = tmp;
        }
        rhead->next = NULL;
        rhead = last;
        //merge the two parts
        while (lhead!=NULL){
            ListNode * tmp = lhead->next;
            if (rhead !=NULL){
                ListNode * tmp1 = rhead->next;
                lhead->next = rhead;
                rhead->next = tmp;
                rhead = tmp1;
            }
            lhead = tmp;
        }
        return;
    }
};