【LeetCode】23. Merge k Sorted Lists

合并k个已合并链表。

思路：先把链表两两合并，直到合并至只有一个链表

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        if(lists.empty()) return NULL;
        int len = lists.size();
        while(len>1){
            for(int i=0;i<len/2;i++){
                lists[i] = mergeTwoLists(lists[i],lists[len-1-i]);
            }
            len = (len+1)/2;   //这个技巧很有用
        }
        return lists.front();
    }
    ListNode* mergeTwoLists(ListNode*l1,ListNode*l2){
        if(NULL==l1) return l2;
        else if(NULL==l2) return l1;
        if(l1->val<l2->val){
            l1->next = mergeTwoLists(l1->next,l2);
            return l1;
        }
        else {
            l2->next = mergeTwoLists(l1,l2->next);
            return l2;
        }
    }
};