main2.cpp

/// Source : https://leetcode.com/problems/two-sum/description/
/// Author : liuyubobobo
/// Time   : 2018-08-21

#include <iostream>
#include <vector>

using namespace std;

/// Definition for singly-linked list.
struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};

class ListOp{

public:
    // 根据n个元素的数组arr创建一个链表, 并返回链表的头
    static ListNode* createLinkedList(const vector<int>& arr){

        if(arr.size() == 0)
            return NULL;

        ListNode* head = new ListNode(arr[0]);
        ListNode* curNode = head;
        for(int i = 1 ; i < arr.size() ; i ++){
            curNode->next = new ListNode(arr[i]);
            curNode = curNode->next;
        }
        return head;
    }

    // 打印以head为头结点的链表信息内容
    static void printLinkedList(ListNode* head){

        ListNode* curNode = head;
        while(curNode){
            cout << curNode->val << " -> ";
            curNode = curNode->next;
        }
        cout << "NULL" << endl;
    }

    // 释放以head为头结点的链表空间
    static void deleteLinkedList(ListNode* head){

        ListNode* curNode = head;
        while(curNode){
            ListNode* delNode = curNode;
            curNode = curNode->next;
            delete delNode;
        }
    }
};


/// Merge Sort Bottom Up
/// Time Complexity: O(nlogn)
/// Space Complexity: O(1)
class Solution {
public:
    ListNode* sortList(ListNode* head) {

        ListNode* dummyHead = new ListNode(-1);
        dummyHead->next = head;
        int sz = 1;
        while(true){
            ListNode *pre = dummyHead, *cur = pre;

            while(cur->next){
                cur = pre;
                for(int i = 0; i < sz && cur->next; i ++, cur = cur->next);

                if(cur->next){
                    ListNode* pre2 = cur;
                    for(int i = 0; i < sz && cur->next; i ++, cur = cur->next);
                    ListNode* next = cur->next, *head2 = pre2->next;
                    pre2->next = NULL, cur->next = NULL;

                    ListNode* tail;
                    pre->next = merge(pre->next, head2, tail);

                    pre = tail;
                    pre->next = next;
                }
                else if(pre == dummyHead){
                    sz = 0;
                    break;
                }
            }

            if(sz == 0) break;
            else sz *= 2;
        }

        ListNode* ret = dummyHead->next;
        delete dummyHead;
        return ret;
    }

private:
    ListNode* merge(ListNode* a, ListNode* b, ListNode* &tail){

        ListNode* dummyHead = new ListNode(-1);
        ListNode *p1 = a, *p2 = b, *p = dummyHead;
        while(p1 && p2)
            if(p1->val < p2->val){
                p->next = p1;
                p1 = p1->next;
                p = p->next;
                p->next = NULL;
            }
            else{
                p->next = p2;
                p2 = p2->next;
                p = p->next;
                p->next = NULL;
            }
        if(p1) p->next = p1;
        if(p2) p->next = p2;

        tail = p;
        while(tail->next) tail = tail->next;

        ListNode* ret = dummyHead->next;
        delete dummyHead;
        return ret;
    }
};


int main() {

    vector<int> arr = {4, 2, 1, 3};
    ListNode* head = Solution().sortList(ListOp::createLinkedList(arr));
    ListOp::printLinkedList(head);
    ListOp::deleteLinkedList(head);

    return 0;
}