leetcode: Merge k Sorted Lists

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

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

思路：

不停的做两两合并就可以了。

class Solution {
public:
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        ListNode *head = NULL, *tail = NULL, *tmp;
        
        while ((l1 != NULL) && (l2 != NULL)) {
            if (l1->val < l2->val) {
                tmp = l1;
                l1 = l1->next;
            }
            else {
                tmp = l2;
                l2 = l2->next;
            }
            
            if (NULL == head) {
                head = tail = tmp;
            }
            else {
                tail->next = tmp;
                tail = tmp;
            }
        }
        
        tmp = (NULL == l1) ? l2 : l1;
        
        if (head != NULL) {
            tail->next = tmp;
            
            return head;
        }
        else {
            return tmp;
        }
    }
    
    ListNode *mergeKLists(vector<ListNode *> &lists) {
        if (0 == lists.size()) {
            return NULL;
        }
        
        ListNode *p = lists[0];
        for (int i = 1; i < lists.size(); ++i) {
            p = mergeTwoLists(p, lists[i]);
        }
        
        return p;
    }
};