[LeetCode]Merge k Sorted Lists

原题链接：http://oj.leetcode.com/problems/merge-k-sorted-lists/

题目描述：

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

题解：

　　这是个典型的分治题，用递归，不断平分，解决merge两个有序链表的小问题即可。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *merge(ListNode* A,ListNode* B){
        if(A==NULL && B==NULL)
            return NULL;
        if(A == NULL)
            return B;
        if(B == NULL)
            return A;
        ListNode *p = A;
        ListNode *q = B;
        ListNode *res = NULL;
        ListNode *pre = NULL;
        if(p->val<=q->val){
            res = p;
            p = p->next;
            pre = res;
        }else{
            res = q;
            q = q->next;
            pre = res;
        }
        while(p!=NULL && q!=NULL){
            if(p->val<=q->val){
                pre->next = p;
                pre = p;
                p = p->next;
            }else{
                pre->next = q;
                pre = q;
                q = q->next;
            }
        }
        while(p!=NULL){
            pre->next = p;
            pre = p;
            p = p->next;
        }
        while(q!=NULL){
            pre->next = q;
            pre = q;
            q = q->next;
        }
        pre->next = NULL;
        return res;
    }
    
    ListNode *mergeKLists(vector<ListNode *> &lists) {
        int size = lists.size();
        if(size == 0)
            return NULL;
        if(size == 1)
            return lists[0];
        vector<ListNode*> A,B;
        for(int i=0; i<size/2; i++){
            A.push_back(lists[i]);
        }
        for(int i=size/2; i<size; i++){
            B.push_back(lists[i]);
        }
        return merge(mergeKLists(A),mergeKLists(B));
    }
};