合并两个排序的链表

题目描述：

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

非递归版：

/*
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 *head = new ListNode(0);
        ListNode *l = head, *p = pHead1, *q = pHead2;
        while(p && q){
            if(p->val < q->val){
                l->next = p;
                l = l->next;
                p = p->next;
            }
            else{
                l->next = q;
                l = l->next;
                q = q->next;
            }
        }
        while(p){
            l->next = p;
            l = l->next;
            p = p->next;
        }
        while(q){
            l->next = q;
            l = l->next;
            q = q->next;
        }
        return head->next;
    }
};

递归版：

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