165. 合并两个排序链表

将两个排序链表合并为一个新的排序链表

您在真实的面试中是否遇到过这个题？

Yes

样例

给出 1->3->8->11->15->null，2->null，返回 1->2->3->8->11->15->null。

/**
 * Definition of ListNode
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 */


class Solution {
public:
    /*
     * @param l1: ListNode l1 is the head of the linked list
     * @param l2: ListNode l2 is the head of the linked list
     * @return: ListNode head of linked list
     */
    ListNode * mergeTwoLists(ListNode * l1, ListNode * l2) {
        // write your code here
        ListNode *pnew, *phead = l1;
        if (l1 == NULL) {
            return l2;
        } else if (l2 == NULL) {
            return l1;
        } else {
            if (l1->val <= l2->val) {
                phead = pnew = l1;
                l1 = l1->next;
            } else {
                phead = pnew = l2;
                l2 = l2->next;
            }
            while (l1 != NULL && l2 != NULL) {
                if (l1->val <= l2->val) {
                    pnew->next = l1;
                    l1 = l1->next;
                    pnew = pnew->next;
                } else {
                    pnew->next = l2;
                    l2 = l2->next;
                    pnew = pnew->next;
                }
            }
            if (l1 != NULL && l2 == NULL) {
                pnew->next = l1;
            }
            if (l2 != NULL && l1 == NULL) {
                pnew->next = l2;
            }
            return phead;
        }
    }
};