# 21 - Merge Two Sorted Lists

解法一 - 簡單暴力法

就建立一個新的 list，依序把比較小的 node 放進去就好。

/**
 * 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* newHead = new ListNode(0);

        ListNode* prev = newHead;
        while(l1 != nullptr && l2 != nullptr) {
            if(l1->val < l2->val) {
                prev->next = l1;
                l1 = l1->next;
            }
            else {
                prev->next = l2;
                l2 = l2->next;
            }
            prev = prev->next;
        }

        prev->next = (l1 == nullptr) ? l2 : l1;

        ListNode* res = newHead->next;
        delete newHead;
        return res;
    }
};

Runtime: 4 ms, faster than 98.91% of C++ online submissions for Merge Two Sorted Lists. Memory Usage: 9 MB, less than 66.39% of C++ online submissions for Merge Two Sorted Lists.