【LeetCode】21. Merge Two Sorted Lists

1.题目描述

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.

将两个有序链表合并为一个新的有序链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。

Example:

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

2.Solutions

递归版本：

public static 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;
    }
}

迭代版本：

public static ListNode mergeTwoListsIterative(ListNode l1, ListNode l2) {
    if(l1 == null) return l2;
    if(l2 == null) return l1;
    ListNode start = new ListNode(0);
    ListNode cur = start;
    while(l1 != null && l2 != null){
        if(l1.val <= l2.val){
            cur.next = l1;
            l1 = l1.next;
        }else{
            cur.next = l2;
            l2 = l2.next;
        }
        cur = cur.next;
    }
    cur.next = l1 == null ? l2 : l1;
    return start.next;
}

（完）