https://leetcode-cn.com/problems/merge-two-sorted-lists/

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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode *p = new ListNode();
        ListNode *ans = p;
        
        while(l1 != nullptr && l2 != nullptr) {
            if(l1.val > l2.val) {
                p -> next = new ListNode(l2 -> val);
                p = p -> next;
                l2 = l2 -> next;
            }
            else {
                p -> next = new ListNode(l1 -> val);
                p = p -> next;
                l1 = l1 -> next;
            }
        }
        while(l1 != nullptr) {

            p -> next = new ListNode(l1 -> val);
            p = p -> next;
            l1 = l1 -> next;
        }
        while(l2 != nullptr) {
            //p -> val = l2 -> val;
            p -> next = new ListNode(l2 -> val);
            p = p -> next;
            l2 = l2 -> next;
        }
        return ans -> next;
    }
};