leetcode: Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first
two lists.

analysis: find a main link and a sub link.

/**
* 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 *pMain, *pSub, *cur, *next;

if (!l1 || !l2) {
return l1 ? l1 : l2;
}

pMain = l1->val < l2->val ? l1 : l2;
pSub = l1->val < l2->val ? l2 : l1;

cur = pMain;
next = cur->next;

while (cur && pSub && next) {
if (cur->val <= pSub->val && next->val >= pSub->val) {
ListNode *tmp = pSub;
pSub = pSub->next;
cur->next = tmp;
tmp->next = next;
} else {
cur = next;
}
next = cur->next;
}

if (pSub) {
cur->next = pSub;
}

return pMain;
}
};