148. Sort List

148. Sort List

/**
 * 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:
    // split O(logn)
    ListNode* sortList(ListNode* head) {
        // head->next 是为了单个节点的情况
        if(!head || !head->next)
            return head;
        auto midPoint = getMid(head);
        auto left = sortList(head);
        auto right = sortList(midPoint);
        return mergeList(left, right);
    }
private:
    // 这里的获取终点函数记得要进行部分调整，要从中间切断这个list
    // O(n)
    ListNode* getMid(ListNode* head)
    {
        ListNode* walker = nullptr;
        ListNode* runner = head;
        while(runner && runner->next)
        {
            walker = !walker ? head : walker->next;
            runner = runner->next->next;
        }
        ListNode* mid = walker->next;
        walker->next = nullptr;
        return mid;
    }
    // O(n)
    ListNode* mergeList(ListNode* l1, ListNode* l2)
    {
        if(!l1)
            return l2;
        if(!l2)
            return l1;
        if(l1->val > l2->val)
        {
            l2->next = mergeList(l2->next, l1);
            return l2;
        }else
        {
            l1->next = mergeList(l1->next, l2);
            return l1;
        }
        return nullptr;
    }
};

T(n) : O(nlogn)

总体是分成许多一半一半的，然后每一小部分merge

优化merge

/**
 * 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 || !head->next)
            return head;
        auto midPoint = getMid(head);
        auto left = sortList(head);
        auto right = sortList(midPoint);
        return mergeList(left, right);
    }
private:
    ListNode* getMid(ListNode* head)
    {
        ListNode* walker = nullptr;
        ListNode* runner = head;
        while(runner && runner->next)
        {
            walker = !walker ? head : walker->next;
            runner = runner->next->next;
        }
        ListNode* mid = walker->next;
        walker->next = nullptr;
        return mid;
    }
    ListNode* mergeList(ListNode* l1, ListNode* l2)
    {
        ListNode dummyHead(0);
        auto prev = &dummyHead;
        while(l1 && l2)
        {
            if(l1->val > l2->val)
            {
                prev->next = l2;
                l2 = l2->next;
            }else
            {
                prev->next = l1;
                l1 = l1->next;
            }
            prev = prev->next;
        }
        if(l1)
            prev->next = l1;
        else
            prev->next = l2;
        return dummyHead.next;
    }
};