leetcode - 23. Merge k Sorted Lists

多路归并

/**
 * 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* l3,*l4;
        if(l1==NULL||l2==NULL)
        return l1==NULL?l2:l1;
        if((l1->val)>(l2->val))
        {
            l3=l2;
            l2=l2->next;
        }else{
            l3=l1;
            l1=l1->next;
        }
        l4=l3;
        while(l1!=NULL&&l2!=NULL)
        {
            if(l1->val>l2->val)
            {
                l4->next=l2;
                l4=l4->next;
                l2=l2->next;
            }else{
                l4->next=l1;
                l4=l4->next;
                l1=l1->next;
            }
        }
        if(l1==NULL)
        {
            l4->next=l2;
        }else{
            l4->next=l1;
        }
        return l3;
    }
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        int n=lists.size(),k;
        if(!n) return NULL;
        while(n>1)
        {
            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];
    }
};