/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode *mergeTwoLists(struct ListNode *list1, struct ListNode *list2) {
    struct ListNode **ppCur;
    for(ppCur = &list1; *ppCur; ppCur = &(*ppCur)->next) {
        if(!list2)
            break;

        while(list2 && list2->val < (*ppCur)->val) {
            struct ListNode *pTmp = list2->next;
            list2->next = *ppCur;
            *ppCur = list2;
            list2 = pTmp;
        }
    }

    if(list2)
        *ppCur = list2;

    return list1;
}