[Leetcode] Sort List

Sort a linked list in O(n log n) time using constant space complexity.

归并排序！用慢指针、快指针来寻找二分位置，这里很容易出现错，要特别注意。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *mergeSort(ListNode *l1, ListNode *l2) {
        ListNode *head = new ListNode(-1);
        ListNode *pos = head;
        while (l1 != NULL && l2 != NULL) {
            if (l1->val < l2->val) {
                pos->next = l1;
                l1 = l1->next;
            } else {
                pos->next = l2;
                l2 = l2->next;
            }
            pos = pos->next;
        }
        if (l1 == NULL) pos->next = l2;
        if (l2 == NULL) pos->next = l1;
        return head->next;
    }
    
    ListNode *sortList(ListNode *head) {
        if (head == NULL || head->next == NULL) return head;
        ListNode *slow = head, *fast = head;
        while (fast != NULL && fast->next != NULL) {
            if (fast->next) fast = fast->next;
            else break;
            if (fast->next) fast = fast->next;
            else break;
            if (slow) slow = slow->next;
        }
        ListNode *head2 = slow->next;
        slow->next = NULL;
        head = sortList(head);
        head2 = sortList(head2);
        return mergeSort(head, head2);
    }
};