143. Reorder List.cpp

/**
 * 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) {
        if(!head) return;
        ListNode* slow = head, *fast = head;
        ListNode* pre = new ListNode(0);
        pre->next = slow;
        while(fast && fast->next){
            pre = pre->next;
            slow = slow->next;
            fast = fast->next->next;
        }
        if(fast) slow = slow->next, pre = pre->next;
        pre->next = NULL;
        ListNode* cur, *next;
        pre = NULL, cur = slow;
        while(cur){
            next = cur->next;
            cur->next = pre;
            pre = cur;
            cur = next;
        }
        ListNode* h1 = head, *h2 = pre, *p1, *p2;
        while(h1 && h2){
            p1 = h1->next;
            p2 = h2->next;
            h1->next = h2;
            h2->next = p1;
            h1 = p1;
            h2 = p2;
        }
    }
};