Merge Two Sorted Lists, 合并两个有序链表

/**
 * 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) {
        //考虑空指针问题
        //采用非递归写法
        if (!l1)
            return l2;
        if (!l2)
            return l1;
        ListNode *p, *q, *r, *pHead;
        p = l1;
        q = l2;
        if (p->val <= q->val)
        {
            pHead = p;
            p = p->next;
        }
        else
        {
            pHead = q;
            q = q->next;
        }
        r = pHead;
        while (1)
        {
            if (!p)
            {
                r->next = q;
                break;
            }
            if (!q)
            {
                r->next = p;
                break;
            }
            if (p->val <= q->val)
            {
                r->next = p;
                r = p;
                p = p->next;
            }
            else
            {
                r->next = q;
                r = q;
                q = q->next;
            }
        }
        return pHead;
    }
};

同样，也可以采用递归写法，见《剑指offer》