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

merge-two-sorted-lists.cpp

/**
 * 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)
    {
        if (l1 == NULL) return l2;
        if (l2 == NULL) return l1;
        
        auto v1 = l1->val;
        auto v2 = l2->val;
        auto root = v1 <= v2 ? l1 : l2;
        v1 <= v2 ? (l1 = l1->next) : (l2 = l2->next);

        auto cur = root;
        while (l1 != NULL && l2 != NULL)
        {
            if (l1->val <= l2->val)
            {
                cur->next = l1;
                l1 = l1->next;
            }
            else
            {
                cur->next = l2;
                l2 = l2->next;
            }
            cur = cur->next;
        }
        cur->next = l1 != NULL ? l1 : l2;

        return root;
    }
};

Runtime: 8 ms, faster than 100.00% of C++ online submissions for Merge Two Sorted Lists.
Memory Usage: 9 MB, less than 99.71% of C++ online submissions for Merge Two Sorted Lists.

Runtime: 8 ms, faster than 100.00% of C++ online submissions for Merge Two Sorted Lists.
Memory Usage: 8.8 MB, less than 100.00% of C++ online submissions for Merge Two Sorted Lists.

• Why?
  auto root = l1->val<= l1->val? l1 : l2;
  l1->val<= l1->val? (l1 = l1->next) : (l2 = l2->next);
  Speed change to 16ms