leetcode [Merge Two Sorted Lists]

解法一：

/** 
 * 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) {  
        ListNode res = new ListNode(0);  
        ListNode temp = res;//记得右移一位  
        while(l1 != null && l2 != null){  
            if(l1.val <= l2.val){  
                temp.next = new ListNode(l1.val);  
                temp = temp.next;  
                l1 = l1.next;  
            }  
            else{  
                temp.next = new ListNode(l2.val);  
                temp = temp.next;  
                l2 = l2.next;  
            }  
        }  
        while(l1 != null){  
            temp.next = new ListNode(l1.val);  
            temp = temp.next;  
            l1 = l1.next;  
        }  
        while(l2 != null){  
            temp.next = new ListNode(l2.val);  
            temp = temp.next;  
            l2 = l2.next;  
        }  
        res = res.next;  
        return res;  
    }  
} 

 解法二（递归）：

/** 
 * 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) {//递归求法  
        ListNode res = null;  
        if(l1 == null){ return l2;}//递归出口  
        if(l2 == null){ return l1;}//递归出口  
        if(l1.val <= l2.val){  
            res = new ListNode(l1.val);  
            res.next = mergeTwoLists(l1.next, l2);  
        }  
        else{  
            res = new ListNode(l2.val);  
            res.next = mergeTwoLists(l1, l2.next);  
        }  
        return res;  
    }  
}