Leetcode-21 Merge Two Sorted Lists (java)

21. Merge Two Sorted Lists

原題目

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.

翻譯

将兩個有序連結清單合并成一個新的連結清單。新的連結清單應該有由這兩個連結清單的頭部拼接而成。

解題思路

按照題意新連結清單有這兩個有序連結清單的頭部拼接而成，是以需要先找到比較兩個連結清單的頭部，

将較小的那個當做新連結清單的頭部，然後同時周遊兩個連結清單進行拼接就行了。

時間複雜度為:O(m+n)

空間複雜度為:O(0)

代碼示例-Java

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1 == null ){
            return l2;
        }

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

        ListNode ln;
        ListNode head;
        if (l1.val < l2.val){
            head = ln = l1;
            l1 = l1.next;
        }else{
            head = ln = l2;
            l2 = l2.next;
        }

        while (l1 != null || l2 != null){
            if (l1 == null){
                ln.next = l2;
                break;
            }else if (l2 == null){
                ln.next = l1;
                break;
            }

            if (l1.val < l2.val){
                ln.next = l1;
                ln = ln.next;
                l1 = l1.next;
            }else{
                ln.next = l2;
                ln = ln.next;
                l2 = l2.next;
            }
        }

        return head;
    }