LeetCode/21. Merge Two Sorted Lists.md

# [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;
    }
};
```