將兩個有序連結串列合併為一個新的有序連結串列並返回。新連結串列是通過拼接給定的兩個連結串列的所有節點組成的。

輸入：1->2->4, 1->3->4
輸出：1->1->2->3->4->4

方案一：歸併中的merge方法

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */

class Solution {
    
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        
        //定義新連結串列頭
        ListNode temp = new ListNode(0);

        //定義三個連結串列的移動指標
        ListNode current1=l1;
        ListNode current2=l2;
        ListNode current3=temp;

        int t=0;

        while(current1 !=null && current2 !=null){
            if(current1.val < current2.val){
                current3.next=current1;
                current3 = current3.next;
                current1=current1.next;
             
            }else{

                current3.next=current2;
                current3 = current3.next;
                current2=current2.next;
                
            }
        }
         
        
        //l2排完,l1未排完  
        while(current1 !=null){
                
                current3.next=current1;
                current3 = current3.next;
                current1=current1.next;
                  
        }
            
        while(current2 !=null){

                current3.next=current2;
                current3 = current3.next;
                current2=current2.next;
        }
        
        return temp.next;
            
        
    }
}
 

方案二：遞迴

        public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
                if(l1==null){
                    return l2;
                }
                if(l2==null){
                    return l1;
                }
                ListNode mergeNode;
                if(l1.val<l2.val){
                    mergeNode = l1;
                    mergeNode.next = mergeTwoLists(l1.next,l2);
                }else{
                    mergeNode = l2;
                    mergeNode.next = mergeTwoLists(l1,l2.next);
                }
                return mergeNode;        
        }