leetcode 21: Merge two ordered linked lists

Title: Merge two sorted linked lists

• Title description:
  Merge two ordered lists into a new ordered list and return it. The new linked list is formed by splicing all the nodes of the given two linked lists.

• Example:

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

/**
 * 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 temp_head(0);  //设置临时头节点temp_head
        ListNode *ptr = &temp_head; //使用pre指针指向temp_head
        while(l1&&l2){  //l1和l2不空时，对它们进行比较
            if(l1->val < l2->val){   //如果l1对应的节点小于l2对应的节点
                ptr->next = l1;   //将ptr与较小的节点进行连接
                l1 = l1->next;
            }
            else{
                ptr->next = l2;
                l2 = l2->next;
            }
            ptr = ptr->next;  //ptr指向新连接的节点
        }
        if(l1){  //如果l1有剩余
            ptr->next = l1;   //将l1接到ptr后
        }
        if(l2){
            ptr->next = l2;  //将l2接到ptr后
        }
        return temp_head.next;
    }
};