148. Sort List

如何求列表的sort？就是用归并排序，好好学习这个方法，递归先求中点，然后求前面的sort，之后求后面的sort，最后merge！

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* merge(ListNode* h1, ListNode* h2){
        ListNode* hn = new ListNode(INT_MIN);
        ListNode* c = hn;

        while(h1 != NULL && h2 != NULL){
            if(h1 -> val < h2 -> val){
                c -> next = h1;
                h1 = h1 -> next;
            }
            else{
                c -> next = h2;
                h2 = h2 -> next;
            }
            c = c -> next;
        }

        if(h1 != NULL) c -> next = h1;
        if(h2 != NULL) c -> next = h2;

        return hn -> next;
    }

    ListNode* sortList(ListNode* head) {
        if(head == NULL || head -> next == NULL) return head;

        ListNode* f = head -> next -> next;
        ListNode* p = head;
        while(f != NULL && f -> next != NULL){
            p = p -> next;
            f = f -> next -> next;
        }
        ListNode* h = sortList(p -> next);
        p -> next = NULL;
        return merge(sortList(head), h);
    }
};