LeetCode之Sort List

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 *采用归并排序法。*/
class Solution {
public:
    ListNode* sortList(ListNode* head) {
        if(head == nullptr || head->next == nullptr) return head;
        ListNode *fast(head), *slow(head);
        while(fast->next && fast->next->next){
            fast = fast->next->next;
            slow = slow->next;
        }
        fast = slow;
        slow = slow->next;
        fast->next = nullptr;
        
        ListNode *l1 = sortList(head);
        ListNode *l2 = sortList(slow);
        return merge2Lists(l1, l2);
    }
    
    ListNode* merge2Lists(ListNode *l1, ListNode *l2){
        if(l1 == nullptr) return l2;
        if(l2 == nullptr) return l1;
        ListNode node(-1);
        ListNode *p(&node);
        while(l1 && l2){
            if(l1->val < l2 -> val){
                p->next = l1;
                l1 = l1->next;
            }
            else{
                p->next = l2;
                l2 = l2->next;
            }
            p = p->next;
        }
        if(l1) p->next = l1;
        else if(l2) p->next = l2;
        return node.next;
    }
};