Leetcode 21. Merge Two Sorted Lists

21. 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

题目链接：https://leetcode.com/problems/merge-two-sorted-lists/description/

题目大意：合并两个有序链表为一个有序链表

解题思路：之前不太会用链表，这题是用来熟悉链表结构体的操作的。

代码：

#include <cstdio>
#include <cstring>
#include <cmath>
#include <iostream>
#include <vector>
#include <string>
#include <queue>
#include <algorithm>
#include <cmath>
#include <map>
#include <stack>
using namespace std;

struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};
 
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode* head = new ListNode(0);
        ListNode* ans = head;
        while(l1 != NULL || l2 != NULL){  
            if(l1 == NULL){
                head -> next = new ListNode(l2 -> val); 
                head = head ->next;
                l2 = l2 -> next;   
                continue;
            }else if(l2 == NULL){
                head -> next = new ListNode(l1 -> val); 
                head = head ->next;
                l1 = l1 -> next;
                continue;   
            }
            int a = l1 -> val;
            int b = l2 -> val;
            if(a > b){
                head -> next = new ListNode(b);
                head = head ->next;
                l2 = l2 -> next;

            }else{
                head -> next = new ListNode(a);
                head = head ->next;
                l1 = l1 -> next;
            }
        }
        ans = ans -> next;
        return ans;
    }
};
int main(void){
    int n,m,x;
	// vector <string> vt;
    scanf("%d%d",&n,&m);
	Solution so;
    ListNode* head1 = new ListNode(0);
    ListNode* l1  = head1;
    for(int i = 0;i < n ; i ++){
        scanf("%d",&x);
        head1 -> next = new ListNode(x);
        head1 = head1 -> next  ;
    }
    ListNode* head2 = new ListNode(0);
    ListNode* l2 = head2;
    for(int i = 0;i < m ; i ++){
        scanf("%d",&x);
        head2 -> next = new ListNode(x);
        head2 = head2 -> next;
    }
    l1 = l1 -> next;
    l2 = l2 -> next;
    ListNode* ans = so.mergeTwoLists(l1,l2);
    while(ans != NULL){
    	cout << ans -> val <<endl;
        ans = ans -> next;
    }
}