[LeetCode]Merge k Sorted Lists

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

这道题是要求归并k个有序单链表。

采用分治法，将任务分成两个子任务，然后递归求子任务，最后回溯：将k个list分成两半，然后继续划分，剩余连个list就合并。

/**
 * 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* h1 = l1;
        ListNode* h2 = l2;
        ListNode* ans = new ListNode(0);
        ListNode* l3 = ans;
        while (h1&&h2){
            if (h1->val <= h2->val){
                ans->next = h1;
                ans = h1;
                h1 = h1->next;
            }
            else{
                ans->next = h2;
                ans = h2;
                h2 = h2->next;
            }
        }
        ans->next = h1 ? h1 : h2;
        return l3->next;
    }

    ListNode *mergeKLists(vector<ListNode *> &lists) {
        int len = lists.size();
        if (!len)
            return NULL;
        int count = 1;
        while (len>1){
            for (int i = 0; i < lists.size()-count; i += (count*2)){
                lists[i] = mergeTwoLists(lists[i], lists[i + count]);
            }
            count*=2;
            len = len / 2 + len%2;
        }
        return lists[0];
    }
};