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

概述： 给定两个链表，让合并成一个链表，而且大小顺序要一定

思路： 遍历两个链表，谁值小，先插谁。

/**
 * 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 *head = new ListNode(-1);
        ListNode *cur = head;
        while(l1 != NULL || l2 != NULL)
        {
            bool flag = (l1 != NULL ? (l1 -> val) : INT_MAX) < (l2 != NULL ? (l2 -> val) : INT_MAX) ? true : false;
            if(flag)
            {
                cur -> next = new ListNode(l1 -> val);
                cur = cur -> next;
                l1 = l1 -> next;
            }
            else
            {
                cur -> next = new ListNode(l2 -> val);
                cur = cur -> next;
                l2 = l2 -> next;
            }
        }
        
        return head -> next;
    }
};