21. Merge Two Sorted Lists

// LeetCode URL: https://leetcode.com/problems/add-two-numbers/

//   Definition for singly-linked list.
class ListNode {
    int val;
    ListNode next;

    ListNode(int x) {
        val = x;
    }
}

/**
 * Iterative Solution
 *
 * Time Complexity: O(min(M, N))
 *
 * Space Complexity: O(1)
 *
 * M = Length of the input list l1. N = Length of the input list l2.
 */
class Solution1 {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1 == null) {
            return l2;
        }
        if (l2 == null) {
            return l1;
        }

        ListNode dummyHead = new ListNode(-1);
        ListNode prevNode = dummyHead;
        ListNode p = l1;
        ListNode q = l2;

        while (p != null && q != null) {
            if (p.val <= q.val) {
                prevNode.next = p;
                p = p.next;
            } else {
                prevNode.next = q;
                q = q.next;
            }
            prevNode = prevNode.next;
            prevNode.next = null;
        }

        prevNode.next = p == null ? q : p;
        return dummyHead.next;
    }
}

/**
 * Recursive Solution
 *
 * Time Complexity: O(min(M, N))
 *
 * Space Complexity: O(min(M, N))
 *
 * M = Length of the input list l1. N = Length of the input list l2.
 */
class Solution2 {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1 == null) {
            return l2;
        }
        if (l2 == null) {
            return l1;
        }

        return mergeTwoListsHelper(l1, l2);
    }

    private ListNode mergeTwoListsHelper(ListNode p, ListNode q) {
        if (p == null) {
            return q;
        }
        if (q == null) {
            return p;
        }
        if (p.val <= q.val) {
            p.next = mergeTwoListsHelper(p.next, q);
            return p;
        } else {
            q.next = mergeTwoLists(p, q.next);
            return q;
        }
    }
}