[LeetCode]021. Merge Two Sorted Lists

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.

Solution:

use a recursion method, two pointers

running time: O(n).

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(l1 == null){
            return l2;
        }
        if(l2 == null){
            return l1;
        }
        ListNode head;
        if(l1.val<l2.val){
            head = l1;
            head.next = mergeTwoLists(l1.next, l2);
        }else{
            head = l2;
            head.next = mergeTwoLists(l1,l2.next);
        }
        return head;
    }
}