16. 合并两个排序的链表

题目：输入两个单调递增的链表，输出两个链表合成后的链表，需要合成后的链表满足单调不减规则。

思路：递归。

/*
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 == NULL)
            return pHead2;
        if(pHead2 == NULL)
            return pHead1;
        ListNode* pMergeHead;
        if(pHead1->val < pHead2->val){
            pMergeHead = pHead1;
            pMergeHead->next = Merge(pHead1->next, pHead2);
        }
        else{
            pMergeHead = pHead2;
            pMergeHead->next = Merge(pHead1, pHead2->next);
        }
        return pMergeHead;
    }
};