题解 | #链表排序#

快速排序

/**
 * struct ListNode {
 *    int val;
 *    struct ListNode *next;
 * };
 */

class Solution {
public:
    /**
     * 
     * @param head ListNode类 the head node
     * @return ListNode类
     */
    ListNode* get_tail(ListNode* head) {
        while(head->next) head = head->next;
        return head;
    }

    ListNode* sortList(ListNode* head) {
        // write code here
        if (!head || !head->next) return head;
        auto left = new ListNode(-1), mid = new ListNode(-1), right = new ListNode(-1);
        auto ltail = left, mtail = mid, rtail = right;
        int val = head->val;
        for (auto p = head; p; p = p->next) {
            if (p->val < val) ltail = ltail->next = p;
            else if (p->val == val) mtail = mtail->next = p;
            else rtail = rtail->next = p;
        }
        // 末尾置0
        ltail->next = mtail->next = rtail->next = NULL;
        left->next = sortList(left->next);
        right->next = sortList(right->next);

        // 链表拼接
        get_tail(left)->next = mid->next;
        get_tail(left)->next = right->next;

        return left->next;
    }
};