【LeetCode】021. 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.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

题解：

　　简单的链表遍历，还可用递归做。

Solution 1

 v/**
  * 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) {
         ListNode dummy(-);
         ListNode* cur = &dummy;

         while(l1 && l2) {
             if (l1->val < l2->val) {
                 cur->next = l1;
                 l1 = l1->next;
             } else {
                 cur->next = l2;
                 l2 = l2->next;
             }
             cur = cur->next;
         }

         cur->next = l1 ? l1 : l2;

         return dummy.next;
     }
 };

　　递归方法

Solution 2

 /**
  * 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) return l2;
         if (!l2) return l1;

         if (l1->val < l2->val) {
             l1->next = mergeTwoLists(l1->next, l2);
             return l1;
         } else {
             l2->next = mergeTwoLists(l1, l2->next);
             return l2;
         }
     }
 };