[leetcode]21. Merge Two Sorted Lists合并两个链表

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.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

题目

合并两个链表

思路

用dummy， 因为需要对头结点进行操作

代码

/* 
Time: O(min(m,n)) 因为一旦l1或l2有一方先遍历完，循环结束
Space: O(1) 因为没有额外多开空间
*/
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        // corner case
        if (l1 == null) return l2;
        if (l2 == null) return l1;
        
        ListNode dummy = new ListNode(-1);
        ListNode p = dummy;
        
        while(l1 != null && l2 != null){
            if (l1.val > l2.val){
                p.next = l2; 
                l2 = l2.next;
            }else{
                p.next = l1; 
                l1 = l1.next;  
            }
            p = p.next;
        }
        p.next = l1 != null ? l1 : l2;
        return dummy.next;
    }
}