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一：归并实现
1.递归写法
2.非递归写法，数组模拟递归（STL里源码思路）
二:插入实现

归并递归写法：

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class Solution {
public:
    ListNode* merge(ListNode* left, ListNode* right) {
        ListNode* result = new ListNode(-1);
        ListNode* r = result;
        while (left != NULL && right != NULL) {
            if (left->val <= right->val) {
                r->next = left;
                r = r->next;
                left = left->next;
            } else {
                r->next = right;
                r = r->next;
                right = right->next;
            }
        }
        if (left != NULL) r->next = left;
        if (right != NULL) r->next = right;
        return result->next;
    }
    ListNode* sortList(ListNode* head) {
        if (head == NULL || head ->next == NULL) return head;
        ListNode* pre = NULL;
        ListNode* fast = head, *slow = head;
        while (fast != NULL && fast->next != NULL) {
            pre = slow;
            fast = fast->next->next;
            slow = slow->next;
        }
        pre->next = NULL;
        ListNode* left = sortList(head);
        ListNode* right = sortList(slow);
        return merge(left, right);
    }
};

归并非递归写法：
第二种着实很难理解，模拟了很久。敬畏敬畏。大道至简，就是这样哇。

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class Solution {
public:
    ListNode* merge(ListNode* l1, ListNode* l2) {
        ListNode* result = new ListNode(-1);
        ListNode* r = result;
        while (l1 && l2) {
            if (l1->val <= l2->val) {
                r->next = l1;
                r = r->next;
                l1 = l1->next;
            } else {
                r->next = l2;
                r = r->next;
                l2 = l2->next;
            }
        }
        if (l1) {
            r->next = l1;
        }
        if (l2) {
            r->next = l2;
        }
        return result->next;
    }
    ListNode* table[64] = {NULL};
    int fill = 0;
    int i = 0;
    ListNode* carry;
    ListNode* sortList(ListNode* head) {
        if  (head == NULL || head->next == NULL) return head;
        while (head != NULL) {
            ListNode* next = head->next;
            head->next = carry;
            carry = head;
            i = 0;
            while (i < fill && table[i]) {
                table[i] = merge(carry, table[i]);
                carry = NULL;
                swap(carry, table[i++]);
            }
            swap(carry, table[i]);
            if(i == fill) fill++;
            head = next;
        }   
         ListNode* result;
        for (int i = 1; i < fill; i++) {
            table[i] = merge(table[i], table[i-1]);
        }
        return table[fill-1];
    }
};

插入排序：

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class Solution {
public:
    ListNode* insertionSortList(ListNode* head) {
        ListNode* result = new ListNode(INT_MIN);
        ListNode* r = result;
        ListNode* next;
        while (head != NULL) {
            next = head->next;
            ListNode* r = result;
            while (r != NULL && r->next != NULL && r->next->val < head->val) {
                r = r->next;
            }
            head->next = r->next;
            r->next = head;
            
            head = next;
        }
        return result->next;
    }
};