#165 Merge Two Sorted Lists

题目描述：

Merge two sorted (ascending) linked lists and return it as a new sorted list. The new sorted list should be made by splicing together the nodes of the two lists and sorted in ascending order.

Have you met this question in a real interview? 

Yes

Example

Given 1->3->8->11->15->null, 2->null , return1->2->3->8->11->15->null.

题目思路：

这题用dummy head很好用，然后用两个pointers l1和l2做比较，哪个小就取哪个。

Mycode（AC = 123ms）：

/**
 * Definition of ListNode
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param ListNode l1 is the head of the linked list
     * @param ListNode l2 is the head of the linked list
     * @return: ListNode head of linked list
     */
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        // write your code here
        ListNode *dummy = new ListNode(0);
        ListNode *tmp = dummy;
        
        while (l1 && l2) {
            if (l1->val <= l2->val) {
                tmp->next = new ListNode(l1->val);
                l1 = l1->next;
            }
            else {
                tmp->next = new ListNode(l2->val);
                l2 = l2->next;
            }
            
            tmp = tmp->next;
        }
        
        if (l1) tmp->next = l1;
        if (l2) tmp->next= l2;
        
        return dummy->next;
    }
};