#yyds幹貨盤點#​ leetcode連結清單操作-和并倆個連結清單

連結清單操作，最好是添加一個虛指針，這樣可以避免頭結點非空判斷。這道題，添加一個虛指針，在講資料合并到新的連結清單上

/**
<p>将兩個升序連結清單合并為一個新的 <strong>升序</strong> 連結清單并傳回。新連結清單是通過拼接給定的兩個連結清單的所有節點組成的。 </p>

<p> </p>

<p><strong>示例 1：</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/10/03/merge_ex1.jpg" style="width: 662px; height: 302px;" />
<pre>
<strong>輸入：</strong>l1 = [1,2,4], l2 = [1,3,4]
<strong>輸出：</strong>[1,1,2,3,4,4]
</pre>

<p><strong>示例 2：</strong></p>

<pre>
<strong>輸入：</strong>l1 = [], l2 = []
<strong>輸出：</strong>[]
</pre>

<p><strong>示例 3：</strong></p>

<pre>
<strong>輸入：</strong>l1 = [], l2 = [0]
<strong>輸出：</strong>[0]
</pre>

<p> </p>

<p><strong>提示：</strong></p>

<ul>
    <li>兩個連結清單的節點數目範圍是 <code>[0, 50]</code></li>
    <li><code>-100 <= Node.val <= 100</code></li>
    <li><code>l1</code> 和 <code>l2</code> 均按 <strong>非遞減順序</strong> 排列</li>
</ul>
<div><div>Related Topics</div><div><li>遞歸</li><li>連結清單</li></div></div><br><div><li>👍 2323</li><li>👎 0</li></div>
*/

//leetcode submit region begin(Prohibit modification and deletion)
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        ListNode dummy = new ListNode(-1);
        ListNode p = dummy;
        while(list1!=null && list2!=null){
            if(list1.val<list2.val){
                p.next=list1;
                list1=list1.next;
            }else{
                p.next=list2;
                list2=list2.next;
            }
            p=p.next;
        }
        if(list1!=null){
            p.next=list1;
        }
        if(list2!=null){
            p.next=list2;
        }

        return dummy.next;
    }
}
//leetcode submit region end(Prohibit modification and deletion)