排序链表(归并排序)

给你链表的头结点 head ，请将其按 升序 排列并返回 排序后的链表 。

 

示例 1：

输入：head = [4,2,1,3]
输出：[1,2,3,4]

示例 2：

输入：head = [-1,5,3,4,0]
输出：[-1,0,3,4,5]

示例 3：

输入：head = []
输出：[]

 

方法一：归并排序

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* sortList(ListNode* head) {
        return sortList(head,nullptr);
    }
    ListNode* sortList(ListNode* head,ListNode* tail) {
        if(head==nullptr) return head;
        //如果只有一个节点
        if(head->next==tail){
            head->next = nullptr;
            return head;
        }
        //找到链表的中点
        ListNode *fast=head,*slow = head;
        while(fast!=tail){
            slow = slow->next;
            fast = fast->next;
            if(fast!=tail){
                fast = fast->next;
            }
        }
        ListNode* mid = slow;
        //归并排序
        return merge(sortList(head,mid),sortList(mid,tail));
    }
    ListNode* merge(ListNode* list1,ListNode* list2){
        ListNode* preHead = new ListNode(-1);
        ListNode* pre = preHead;
        ListNode* head1 = list1;
        ListNode* head2 = list2;
        while(head1&&head2){
            if(head1->val<=head2->val){
                pre->next = head1;
                pre = head1;
                head1=head1->next;
            }else{
                pre->next = head2;
                pre = head2;
                head2=head2->next;
            }
        }
        if(head1){
            pre->next = head1;
        }else{
            pre->next = head2;
        }
        return preHead ->next;
    }
};

 

 

方法二：使用set集合默认按照key进行排序

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* sortList(ListNode* head) {
        if(head==nullptr) return head;
        set<pair<int,ListNode*>> st;
        ListNode *node = head;
        while(node){
            st.insert({node->val,node});
            node = node->next;
        }
        for(auto it = st.begin();it!=st.end();it++){
            auto temp = next(it);
            if(temp==st.end()){
                it->second->next = nullptr;
            }else{
                it->second->next = temp->second;
            }
        }
        return st.begin()->second;
    }
};