[LeetCode] 23. merge k sorted lists

• Problems: merge k 個 sorted 的 lists
• Solution:
• 延續 21 merge two lists, iteratively 將 k 個中的 lists 兩兩 merge 即可

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeLists(ListNode *l1, ListNode *l2)
    {
        // ref to problem 22, merge two sorted list
        if (NULL == l1) return l2;
        if (NULL == l2) return l1;
        
        if (l1->val < l2->val) {
            l1->next = mergeLists(l1->next, l2);
            return l1;
        } else {
            l2->next = mergeLists(l1, l2->next);
            return l2;
        }
    }

    ListNode* mergeKLists(vector& lists) {
        // merge 2 by 2 lists
        size_t iters = lists.size() / 2;
        while (iters > 0) {
            iters = lists.size() / 2;
            for (size_t i = 0; i < iters; i++){
                lists[i] = mergeLists(lists[i], lists[lists.size() - 1 - i]);
            }

            lists.erase(lists.begin() + iters + (1 == lists.size() % 2), lists.end());
        }
        return (lists.size() > 0) ? lists[0] : NULL;
    }
};