21. Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

按序合并两个链表

添加两个结点，一个是首结点，一个用来遍历。

/** * 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) {        if (l1 == NULL) return l2;        if (l2 == NULL) return l1;        ListNode *head, *cur;        if (l1->val < l2->val)        {            head=l1;            l1 = l1->next;        }        else        {            head = l2;            l2 = l2->next;        }        cur = head;        while (l1 && l2)        {            if (l1->val < l2->val)            {                cur->next = l1;                l1 = l1->next;            }            else            {                cur->next = l2;                l2 = l2->next;            }            cur = cur->next;        }        if (!l2) cur->next = l1;        else cur->next = l2;        return head;    }};