快速排序单链表

不修改val的值，只改变链表结构，时间复杂度O (nlgn), 额外空间O(lgn)

class Solution {
public:
    ListNode* getTail(ListNode *head){
        while(head->next){
            head = head->next;
        }
        return head;
    }

    ListNode* quickSortList(ListNode* head) {
    
        // 2. 空节点或单节点
        if(!head || !head->next) return head;
    
    
        // 3. 单链表表头为helper
        ListNode *lhead = new ListNode(-1), *mhead = new ListNode(-1), *rhead = new ListNode(-1);
        auto left = lhead, mid = mhead, right = rhead;
    
        // 4.遍历链表
        int x = head->val;
        for(auto p = head; p ; p = p->next){
            if (p->val < x)
                left = left->next = p; // 5. 插入元素 + 移动指针
            else if (p->val == x)
                mid = mid->next = p;
            else
                right = right->next = p;
        }
    
        // 得到三个链表，递归左边和右边链表
        left->next = mid->next = right->next = nullptr; // 6.各个链表尾部置null
    
        //7. 只需遍历左区间 与 右区间
        lhead->next = quickSortList(lhead->next);
        rhead->next = quickSortList(rhead->next);
    
        //8. 链表拼接
        getTail(lhead)->next = mhead->next;
        getTail(mhead)->next = rhead->next;
    
    
        // 9. 专门的re
        auto re = lhead->next;
    
        // 10. 内存回收
        delete lhead;
        delete mhead;
        delete rhead;
    
        return re;
    }
};