LeetCode.21合并两个有序链表

方法一：递归

 

 

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

方法二：迭代法

class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        //新建一个头结点用来存放结果
        ListNode head = new ListNode(0);
        ListNode res = head;
        while(l1 != null && l2 != null){
            if(l1.val <= l2.val){
                res.next = l1;
                l1 = l1.next;
                //System.out.println(res.val);
            }else{
                res.next = l2;
                l2 = l2.next;
            }
            res = res.next;
        }
        res.next = l1 == null ?  l2 :  l1;
        return head.next;
    }
}