21. Merge Two Sorted Lists

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Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

思路：跟merge two arrays很像。一一比较大小，小的用dummy node接上，再继续找，继续比较。最后检查下两个list，如果两个list都空了，那就完成了。任何一个linkedlist不空，就把之前建立的接到那个linkedlist搜索到的node上。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1==null)
        {
            return l2;
        }
        if(l2==null)
        {
            return l1;
        }
        ListNode dummy=new ListNode(-1);
        ListNode node=dummy;
        while(l1!=null&&l2!=null)
        {
            if(l1.val<l2.val)
            {
                node.next=l1;
                l1=l1.next;
            }
            else
            {
                node.next=l2;
                l2=l2.next;
            }
            node=node.next;
        }
        if(l1!=null)
        {
            node.next=l1;
        }
        if(l2!=null)
        {
            node.next=l2;
        }
        return dummy.next;
    }
}

 

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