【Lintcode】165. Merge Two Sorted Lists

题目地址：

https://www.lintcode.com/problem/merge-two-sorted-lists/description

给定两个单调增的链表，要求将它们合并成一个单调增的链表。代码如下：

public class Solution {
    /**
     * @param l1: ListNode l1 is the head of the linked list
     * @param l2: ListNode l2 is the head of the linked list
     * @return: ListNode head of linked list
     */
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        // write your code here
        ListNode dummy = new ListNode(0), cur = dummy;
        
        while (l1 != null || l2 != null) {
            if (l1 == null) {
                cur.next = l2;
                break;
            } else if (l2 == null) {
                cur.next = l1;
                break;
            } else {
                if (l1.val <= l2.val) {
                    cur.next = l1;
                    l1 = l1.next;
                } else {
                    cur.next = l2;
                    l2 = l2.next;
                }
                cur = cur.next;
            }
        }
        
        return dummy.next;
    }
}

class ListNode {
    int val;
    ListNode next;
    ListNode(int x) {
        val = x;
        next = null;
    }
}

时间复杂度 $O(n)$，空间 $O(1)$。