23. 合并K个升序链表

/**
 * 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* merge(ListNode* a,ListNode * b)
    {
        ListNode *head = nullptr;
        if(a == nullptr){
            return b;
        }
        if(b == nullptr){
            return a;
        }

        if(a->val > b->val){
            head = b;
            b = b->next;
        }
        else{
            head = a;
            a = a->next;
        }
        ListNode *p = head;
        while(a!=nullptr && b!=nullptr)
        {
            if(a->val > b->val){
                p->next = b;
                p = b;
                b = b->next;
            }
            else{
                p->next = a;
                p = a;
                a = a->next;
            }
        }

        if(a!=nullptr){
            p->next = a;
        }
        if(b!=nullptr){
            p->next = b;
        }
        return head;
    }


    ListNode* mergeLink(vector <ListNode*> &lists, int l, int r) {
        if (l == r) return lists[l];
        if (l > r) return nullptr;
        int mid = (l + r) >> 1;
        ListNode *left = mergeLink(lists, l, mid);
	    ListNode *right = mergeLink(lists, mid + 1, r);
        return merge(left,right);
    }

    ListNode* mergeKLists(vector<ListNode*>& lists) 
    {
        int i = 0;
        int j = lists.size()-1;
        int mid = (i+j)/2;
        return mergeLink(lists,i,j);
    }
};