8道链表常考题-排序链表归并

​​https://leetcode-cn.com/problems/merge-k-sorted-lists/​​

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

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2){
        ListNode newhead(0);
        ListNode* newtail = &newhead;
        while(l1 && l2){
            if(l1->val < l2->val){
                newtail->next = l1;
                l1 = l1->next;
            }else{
                newtail->next = l2;
                l2 = l2->next;
            }
            newtail = newtail->next;
        }
        if(l1){
            newtail->next = l1;
        }
        if(l2){
            newtail->next = l2;
        }
        return newhead.next;
    }
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        //截止条件
        if(lists.size() == 0) return NULL;
        if(lists.size() == 1) return lists[0];
        if(lists.size() == 2) return mergeTwoLists(lists[0],lists[1]);
        
        //拆分
        int mid = lists.size()/2;
        std::vector<ListNode*> sub1_lists;
        std::vector<ListNode*> sub2_lists;
        for(int i = 0; i < mid; i++){
            sub1_lists.push_back(lists[i]);
        }
        for(int i = mid; i < lists.size(); i++){
            sub2_lists.push_back(lists[i]);
        }
        ListNode* l1 = mergeKLists(sub1_lists);
        ListNode* l2 = mergeKLists(sub2_lists);
        
        return mergeTwoLists(l1,l2);
    }
};

class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        if (lists.size() == 0) return nullptr;
        int n = lists.size();
        while (n > 1) {
            int k = (n + 1) / 2;
            for (int i = 0; i < n / 2; ++i) {
                lists[i] = mergeTwoLists(lists[i], lists[i + k]);
            }
            n = k;
        }
        return lists[0];
    }
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode dummy(-1);
        ListNode *p = &dummy;
        while(1){
            if(l1==nullptr){
                p->next = l2;
                break;
            }
            if(l2==nullptr){
                p->next = l1;
                break;
            }
            if(l1->val < l2->val){
                p->next = l1;
                l1 = l1->next;
                p = p->next;
            }else{
                p->next = l2;
                l2 = l2->next;
                p = p->next;
            }
        }
        return dummy.next;
    }
};