排序链表(LeetCode)

class Solution {
public:
    // 归并排序
    ListNode* merge(ListNode* a, ListNode* b) {
        if (a == NULL) {
            return b;
        }
        if (b == NULL) {
            return a;
        }
        
        // 这里实质上就是将两个有序链表链接起来，如1->3->5 和 2->8->10 链成 1->2->3->5->8->10
        ListNode* new_head = NULL;
        ListNode* cur = NULL;
        if (a->val <= b->val) {
            new_head = a;
            a = a->next;
        } else {
            new_head = b;
            b = b->next;
        }
        cur = new_head;
        
        while(a && b) {
            if (a->val <= b->val) {
                cur->next = a;
                cur = a;
                a = a->next;
            } else {
                cur->next = b;
                cur = b;
                b = b->next;
            }
        }
        if(a) {
            cur->next = a;
        }
        if (b) {
            cur->next = b;
        }
        return new_head;
    }
    ListNode* sortList(ListNode* head) {
        // 归并排序
        if (head == NULL || head->next == NULL) {
            return head;
        }
        ListNode* first = head;
        ListNode* mid = getMid(head);
        ListNode* second = mid->next;
        mid->next = NULL;    //将链表分为两段
        first = sortList(first);
        second = sortList(second);
        return merge(first, second);
    }
    
    //将链表平分为两段，返回第一段末尾   例如：5个点返回2号点，6个点返回3号点
    ListNode* getMid(ListNode* head) {
        ListNode* slow = head;
        ListNode* fast = head->next;
        while(fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
        }
        return slow;
    }
};