21.Merge Two Sorted Lists

Solution 1: Iteration O(m+n)

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode sentinel = new ListNode(-1);
        ListNode tmp = sentinel;
        while(l1!=null && l2!=null){
            if(l1.val <= l2.val){
                tmp.next = l1;
                l1 = l1.next;
            }
            else{
                tmp.next = l2;
                l2 = l2.next;
            }
            tmp = tmp.next;
        }
        if(l1 == null){
            tmp.next = l2;
        }
        else{
            tmp.next = l1;
        }
        return sentinel.next;
    }
}

Solution 2-Recursion

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
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;
        }
    }
}