【LeetCode】21. Merge Two Sorted Lists

题目：

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.

提示：

此题其实和MergeSort的Merge部分的步骤是类似的。

代码：

/**
 * 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 pre_head(0);
        ListNode *node = &pre_head;
        while (l1 && l2) {
            if (l1->val < l2->val) {
                node->next = l1;
                l1 = l1->next;
            } else {
                node->next = l2;
                l2 = l2->next;
            }
            node = node->next;
        }
        node->next = l1 ? l1 :l2;
        return pre_head.next;
    }
};