28-Merge Two Sorted Lists

easy
21. Merge Two Sorted Lists
Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.
思路：利用map映射，值->指针集合，然后按值从小到大将指针链接起来。时间复杂度O(m+n),空间O(m+n)

/**
 * 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) {
        map<int,vector<ListNode*> > maps;
        push_into_map(l1,maps);
        push_into_map(l2,maps);
        ListNode *now=NULL,*pre=NULL,*head;
        map<int,vector<ListNode*> >::const_iterator it = maps.begin();
        if(it!=maps.end()){
                head = (it->second)[0];
        }
        else head = NULL;
        while(it!=maps.end())
        {
            cout<<(it->second).size()<<endl;
            for(int k=0;k<(it->second).size();++k)
            {
                pre = now;
                now = (it->second)[k];
                if(pre!=NULL)pre->next = now;
            }
            it++;
        }
        return head;
    }
    void push_into_map(ListNode* l,map<int,vector<ListNode*> > &maps)
    {
        while(l!=NULL)
        {
            maps[l->val].push_back(l);
            l= l->next;
        }
    }
};