Merge Two Sorted Lists

🧩 Problem Statement

Merge two sorted linked lists and return the merged list as a new sorted linked list.

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✍️ Function Signature

public ListNode mergeTwoLists(ListNode l1, ListNode l2)

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🚀 Recursive Approach

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    if (l1 == null) return l2;
    if (l2 == null) return l1;
    if (l1.val < l2.val) {
        l1.next = mergeTwoLists(l1.next, l2);
        return l1;
    } else {
        l2.next = mergeTwoLists(l1, l2.next);
        return l2;
    }
}

• Time Complexity: O(n + m), where n and m are the lengths of the two lists.
• Space Complexity: O(n + m) due to recursion stack.

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🧰 Iterative Alternative

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    ListNode dummy = new ListNode(-1);
    ListNode current = dummy;

    while (l1 != null && l2 != null) {
        if (l1.val < l2.val) {
            current.next = l1;
            l1 = l1.next;
        } else {
            current.next = l2;
            l2 = l2.next;
        }
        current = current.next;
    }

    current.next = (l1 != null) ? l1 : l2;

    return dummy.next;
}

• Time Complexity: O(n + m)
• Space Complexity: O(1) (iterative, no extra space)

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🧪 Sample ListNode Definition & Test Cases

public class MergeSortedListsTest {
    public static void main(String[] args) {
        ListNode l1 = buildList(new int[]{1, 3, 5});
        ListNode l2 = buildList(new int[]{2, 4, 6});

        ListNode merged = new MergeSortedLists().mergeTwoLists(l1, l2);
        printList(merged);  // Output: 1 -> 2 -> 3 -> 4 -> 5 -> 6

        ListNode l3 = buildList(new int[]{});
        ListNode l4 = buildList(new int[]{0});
        printList(new MergeSortedLists().mergeTwoLists(l3, l4));  // Output: 0
    }

    private static ListNode buildList(int[] values) {
        ListNode dummy = new ListNode(-1);
        ListNode current = dummy;
        for (int val : values) {
            current.next = new ListNode(val);
            current = current.next;
        }
        return dummy.next;
    }

    private static void printList(ListNode head) {
        while (head != null) {
            System.out.print(head.val);
            if (head.next != null) System.out.print(" -> ");
            head = head.next;
        }
        System.out.println();
    }
}

class MergeSortedLists {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1 == null) return l2;
        if (l2 == null) return l1;
        if (l1.val < l2.val) {
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        } else {
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        }
    }
}

class ListNode {
    int val;
    ListNode next;
    ListNode(int val) { this.val = val; }
}

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📝 Notes

• 🧠 Use recursion for elegant and concise solutions on small or medium inputs.
• ⚙️ Use iteration for better performance on large lists (space-efficient).
• ✅ Always handle null edge cases at the start of your implementation.