LeetCode23:Merge k Sorted Lists

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

/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
//采用分治法
class Solution {
public:
ListNode *mergeKLists(vector<ListNode *> &lists) {
int n = lists.size();
if (n == 0) return NULL;
while (n > 1)
{
int k = (n + 1) / 2;
for (int i = 0; i < n / 2; i++)
lists[i] = mergeTwoLists(lists[i], lists[i + k]);
n = k;
}
return lists[0];
}

//merge two sorted lists
ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
if (l1 == NULL) return l2;
if (l2 == NULL) return l1;
ListNode *head = new ListNode(-1);
ListNode *p = head;
while (l1 != NULL && l2 != NULL) {
if (l1->val > l2->val)
{
p->next = l2;
l2 = l2->next;
}
else
{
p->next = l1;
l1 = l1->next;
}
p = p->next;
}

if (l1 == NULL)
{
p->next = l2;
}
else if (l2 == NULL)
{
p->next = l1;
}
return head->next;
}
};