剑指offer-合并两个排序的链表

从小的开始作为头结点，然后比较选择较小的进行链接

需要注意头结点确定的时候就要保存下来，因为后面会一直变化往后链接新元素

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
            val(x), next(NULL) {
    }
};*/
class Solution {
public:
    ListNode* Merge(ListNode* pHead1, ListNode* pHead2)
    {
        if(!pHead1) return pHead2;
        if(!pHead2) return pHead1;
        
        ListNode *ph = NULL, *ph1, *ph2;
        ph1 = pHead1, ph2 = pHead2;
        if(ph1->val < ph2->val){
            ph = ph1;
            ph1 = ph1->next;
        }
        else{
            ph = ph2;
            ph2 = ph2->next;
        }
        ListNode *tmp = ph;
        while(ph1 && ph2){
            if(ph1->val < ph2->val){
                ph->next = ph1;
                ph = ph->next;
                ph1 = ph1->next;
            }
            else{
                ph->next = ph2;
                ph = ph->next;
                    ph2 = ph2->next;
            }
        }
        if(ph1){
            ph->next = ph1;
        }
        else if(ph2){
            ph->next = ph2;
        }
        
        return tmp;
    }
};