From b4c62b12739f98544ad8cbbcc650d3cb0bb93548 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E5=94=90=E6=A0=91=E6=A3=AE?= <14021051@buaa.edu.cn>
Date: Wed, 19 Sep 2018 18:46:03 +0800
Subject: [PATCH] Create 21. Merge Two Sorted Lists.md
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+# [21. Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/description/)
+# 思路
+合并两个已有序的链表，注意题目给的链表没有头结点，所以为了操作方便可以自己设一个头结点，最后返回头结点的下一个节点即可。两个链表的工作指针就用传进来的l1和
+l2即可。
+# C++
+```
+/**
+ * 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 *res = new ListNode(0); // 自己设一个头结点
+ ListNode *pre = res; // pre代表已排好序的链表的最后一个节点
+ while(l1 && l2){
+ if(l1 -> val <= l2 -> val){
+ pre -> next = l1;
+ l1 = l1 -> next;
+ }
+ else{
+ pre -> next = l2;
+ l2 = l2 -> next;
+ }
+ pre = pre -> next; // pre后移
+ }
+ // 跳出循环时，l1和l2其中一个是NULL
+ if(l1) pre -> next = l1;
+ else pre -> next = l2;
+
+ // 释放头结点
+ ListNode *head = res;
+ res = head -> next;
+ head -> next = NULL;
+ delete head;
+
+ return res;
+ }
+};
+```