二路归并。
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
ListNode* head = new ListNode(-1);
ListNode* cur = head;
while (l1 && l2) {
if(l1->val < l2->val) {
cur->next = l1;
cur = l1;
l1 = l1->next;
} else {
cur->next = l2;
cur = l2;
l2 = l2->next;
}
}
if (l1) cur->next = l1;
if (l2) cur->next = l2;
return head->next;
}
};