[LeetCode] Merge k Sorted Lists

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

类似于Merge Sort的方法做k-1次

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *merge(ListNode *node1, ListNode *node2)
{
if (node1 == NULL)
return node2;

if (node2 == NULL)
return node1;

ListNode *head = NULL;
ListNode *curNode = NULL;
ListNode *p = node1;
ListNode *q = node2;

while(p && q)
{
ListNode *node;
if (p->val < q->val)
{
node = p;
p = p->next;
}
else
{
node = q;
q = q->next;
}

if (head == NULL)
{
head = curNode = node;
}
else
{
curNode->next = node;
node->next = NULL;
curNode = node;
}
}

if (p)
{
curNode->next = p;
}
else if (q)
{
curNode->next = q;
}

return head;
}

ListNode *mergeKLists(vector<ListNode *> &lists) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
if (lists.size() == 0)
return NULL;

ListNode *head = lists[0];

for(int i = 1; i < lists.size(); i++)
head = merge(head, lists[i]);

return head;
}
};