MergeLists.java


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import java.lang.*;
import java.util.*;
import node.*;

class MergeLists {
	public static ListNode mergeTwoLists(ListNode list1, ListNode list2) {
		if (list1 == null && list2 == null)
			return new ListNode();
		else if (list1 == null)
			return list2;
		else if (list2 == null)
			return list1;

		ListNode curr1 = list1;
		ListNode curr2 = list2;

		ListNode head;
		if (curr1.val < curr2.val) {
			head = curr1;
			curr1 = curr1.next;
		} else {
			head  = curr2;
			curr2 = curr2.next;
		}

		ListNode node = head;
		while (curr1 != null && curr2 != null) {
			if (curr1.val < curr2.val) {
				node.next = curr1;
				curr1 = curr1.next;
			} else {
				node.next = curr2;
				curr2 = curr2.next;
			}
			node = node.next;
		}

		if (curr1 == null) {
			node.next = curr2;
		} else {
			node.next = curr1;
		}

		return head;
	}

	public static void main(String[] args) {
		ListNode second1 = new ListNode(4, null);
		ListNode first1 = new ListNode(2, second1);
		ListNode list1 = new ListNode(1, first1);

		ListNode second2 = new ListNode(4, null);
		ListNode first2 = new ListNode(3, second2);
		ListNode list2 = new ListNode(1, first2);

		ListNode curr = mergeTwoLists(list1, list2);
		while (curr != null) {
			System.out.println(curr.val);
			curr = curr.next;
		}
	}
}