21. Merge Two Sorted Lists

21. Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

My Solution:

• 新建dummy

• l1和l2比大小, 哪个小哪个先加到dummy上去 (merge)

• 返回dummy.next

class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {

        if (l1 == null) return l2;
        if (l2 == null) return l1;

        ListNode dummy = new ListNode(0);
        ListNode curr = dummy;
        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                curr.next = l1;
                l1 = l1.next;
            } else {
                curr.next = l2;
                l2 = l2.next;
            }
            curr = curr.next;
        }

        if (l1 == null) curr.next = l2;
        else curr.next = l1;
        return dummy.next;
    }
}

• recursive version

class Solution {
    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;
        }
    }
}