leetcode Sort List

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *sortList(ListNode *head)
    {
        if (!head || !head -> next) return head;
        ListNode *fast = head, *slow = head;
        
        while(fast -> next && fast -> next -> next)
        {
            fast = fast -> next -> next;
            slow = slow -> next;
        }
        fast = slow;
        slow = slow -> next;
        fast -> next = NULL;
        
        fast = sortList(head);
        slow = sortList(slow);
        return mergeList(fast, slow);
    }
    
    ListNode  *mergeList(ListNode *l1, ListNode *l2)
    {
        if (!l1) return l2;
        if (!l2) return l1;
        
        ListNode *pre = new ListNode (0);
        ListNode *mer = pre;
        
        while(l1 && l2)
        {
            if (l1 -> val < l2 -> val)
            {
                mer -> next = l1;
                l1 = l1 -> next;
                mer = mer -> next;
            }
            else
            {
                mer -> next = l2;
                l2 = l2 -> next;
                mer = mer -> next;
            }
        }
        
        if (l1)
            mer -> next = l1;
        if (l2)
            mer -> next = l2;
            
        mer = pre -> next;
        delete pre;
        return mer;
    }
};

//非递归，空间复杂度为常量
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *sortList(ListNode *head) {
    if (head == NULL)
        return head;
    ListNode *leftList, *rightList, *tail, *temp;
     
    //每次归并的次数,当归并次数为1时则表示没有进行归并，说明归并完成
    int mergeCount;
    int widthSize = 1;      //每次归并链表的宽度,初始值为1,(2,4,8,....)
    int leftSize ;       //左链表大小
    int rightSize ;
 
    while (true)    //退出条件为mergeCount = 0
    {
        //tail指向当前已归并链表最后一个节点，每次归并开始前都要清空
        //每次归并开始前leftList指向最新的head,归并次数置0
        mergeCount = 0;
        tail = NULL;
        leftList = head;
        rightList = NULL;
 
 
        //每次两个链表归并完成后leftList会指向下一个段的左链表
        //如果没有下一个左链表，则一次归并完成，所以以leftList
        //作为判断条件
        while (leftList)
        {
            ++mergeCount;   //每次归并的趟数，到1时表示没有归并
            //找到rightList的位置,顺便算出leftSize大小
            rightList = leftList;   //从左链表开始向后滑动
            leftSize  = 0;
            rightSize = 0;
            for (int i = 0; i < widthSize; ++i)
            {
                if (rightList)
                {
                    ++leftSize;
                    rightList = rightList->next;
                }
                else
                    //最后一个左链表不够一个区间长度，但链表已遍历完,完成查找
                    break;
            }
 
            //找到两个需要归并的链表的头结点，以及大小
            rightSize = widthSize;      //按最大算,判断是否遍历完时加上指针
            //区间长度作为归并循环条件,归并一个区间-1，直至两个区间都排完
            while (leftSize > 0 || ((rightSize > 0)&& rightList))
            {
                //temp保存临时更小的那个节点，tail始终保存已排好的链尾
                //首先要考虑一个区间是否已排完，因为如果区间已排完，leftList
                //或者rightList由于会后移一个单位将指向另一个区间
                if (leftSize == 0)
                {
                    //即((rightSize > 0)&& rightList)为真
                    temp = rightList;
                    rightList = rightList->next;
                    --rightSize;
                }
                else if (rightSize == 0 || rightList == NULL)
                {
                    //leftSize不为0,注意右链表归并完rightList指针会
                    //移到后面，即下一个归并对的最左一个元素
                    temp = leftList;
                    leftList = leftList->next;
                    --leftSize;
                }//剩下的情况就是两个归并对剩余待归并元素都不为空
                else if (leftList->val <= rightList->val)
                {
                    temp = leftList;
                    leftList = leftList->next;
                    --leftSize;
                }
                else if (leftList->val > rightList->val)
                {
                    temp = rightList;
                    rightList = rightList->next;
                    --rightSize;
                }
 
                //找到更小的一个元素，改变链表指向更小的元素
                if (tail == NULL)
                {
                    //第一个找出的元素,定头结点
                    head = temp;
                    tail = temp;
                }
                else
                {
                    tail->next = temp;
                    tail = tail->next;   //tail永远指向最后一个排好序的节点
                }       
 
            }//while (leftSize > 0 || ((rightSize > 0)&& rightList))
 
            //一个归并对完成，前面已经提过rightList指向下一个归并对的最左节点
            leftList = rightList;   
            tail->next = leftList;
             
        }//while (leftList)
 
        tail->next = NULL;      //tail指向最后一个节点，一趟归并完然后把tail的next设为空
         
        //归并完成判断
        if (mergeCount <= 1)
            return head;
        widthSize *=2;
    }
 
    return NULL;
    }