merge-two-sorted-lists.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
import java.util.*;
public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode res = new ListNode(0);
        ListNode head = res;
        while (l1 != null && l2 != null) {
            if (l1.val > l2.val) {
                res.next = l2;
                l2 = l2.next;
            } else {
                res.next = l1;
                l1 = l1.next;
            }
            res = res.next;
        }
        
        while (l1 != null) {
            res.next = l1;
            l1 = l1.next;
            res = res.next;
        }
        while (l2 != null) {
            res.next = l2;
            l2 = l2.next;
            res = res.next;
        }
        return head.next;
    }
    
    // Recursively
    public ListNode mergeTwoListsRecursively(ListNode l1, ListNode l2) {
        if (l1 == null) return l2;
        if (l2 == null) return l1;
        if (l1.val < l2.val) {
            l1.next = mergeTwoListsRecursively(l1.next, l2);
            return l1;
        } else {
            l2.next = mergeTwoListsRecursively(l1, l2.next);
            return l2;
        }
    }
}