LeetCode --- 21. Merge Two Sorted Lists

题目链接：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.

这道题的要求是将两个已经排好序的链表合并成1个有序链表。

这道题的思路比较简单，主要就是考察链表的处理。这里还是像Remove Nth Node From End of List一样先在链表前面加头，这样可以免去对链表头的特殊处理。接下来就是每次取两个链表前面较小的元素，直到有链表到达结尾。最后再将没有到达结尾的链表直接链接到合并的链表的后面，并返回合并后链表头后面的节点指针即可。

时间复杂度：O(n)

空间复杂度：O(1)

 1 class Solution 2 { 3 public: 4     ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) 5     { 6         ListNode *h = new ListNode(0), *p = h; 7          8         while(l1 != NULL && l2 != NULL) 9         {10             if(l1 -> val < l2 -> val)11             {12                 p -> next = l1;13                 l1 = l1 -> next;14             }15             else16             {17                 p -> next = l2;18                 l2 = l2 -> next;19             }20             21             p = p -> next;22         }23         24         if(l1 != NULL)25             p -> next = l1;26         if(l2 != NULL)27             p -> next = l2;28         29         return h -> next;30     }31 };

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