[Leetcode] #23 Merge k Sorted Lists

Discription:

Solution:

class Solution {
public:
ListNode* merge(ListNode *left, ListNode *right){
ListNode node(-1);
ListNode *cur = &node;
while (left && right){
if (left->val < right->val){
cur->next = left;
left = left->next;
}
else{
cur->next = right;
right = right->next;
}
cur = cur->next;
}
cur->next = left ? left : right;
return node.next;
}

ListNode* merge1(ListNode *head1, ListNode* head2){
if (!head1) return head2;
if (!head2) return head1;
if (head1->val<head2->val){
head1->next = merge(head1->next, head2);
return head1;
}
else{
head2->next = merge(head1, head2->next);
return head2;
}
}

ListNode* mergeKLists(vector<ListNode*>& lists, int left, int right) {
if (left>right)  return NULL;  //注释1，去掉这行见注释2
if (left == right) return lists[left];
if (left<right){
int mid = (left + right) >> 1;
ListNode *head1 = mergeKLists(lists, left, mid);
ListNode *head2 = mergeKLists(lists, mid + 1, right);
return merge(head1, head2);
}
}

ListNode* mergeKLists(vector<ListNode*>& lists) {
return mergeKLists(lists, 0, lists.size() - 1);  //注释2，加上 if (lists.empty())  return NULL;
}

ListNode *mergeKLists2(vector<ListNode *> &lists) {
ListNode *p, *p1, *p2;
while (lists.size()>1){
p1 = lists.back();
lists.pop_back();
p2 = lists.back();
lists.pop_back();
p = merge(p1, p2);
lists.insert(lists.begin(), p);
}
return lists.size() == 1 ? lists[0] : NULL;

/* the following method is not fast enough! */
/*
ListNode* pHead = NULL;
for (int i=0; i<lists.size(); i++){
pHead = mergeTwoLists(pHead, lists[i]);
}
return pHead;
*/
}
};