【剑指25】合并两个排序的链表

题目描述

输入两个单调递增的链表，输出两个链表合成后的链表，当然我们需要合成后的链表满足单调不减规则。

分析

1.递归

    public static ListNode Merge(ListNode list1, ListNode list2) {
        if (list1 == null) return list2;
        if (list2 == null) return list1;

        if (list1.val < list2.val) {
            list1.next = Merge(list1.next, list2);
            return list1;
        } else {
            list2.next = Merge(list1, list2.next);
            return list2;
        }

    }

2.非递归：注意保存第一个节点作为返回结果

   public static ListNode Merge(ListNode list1, ListNode list2) {
        if (list1 == null) return list2;
        if (list2 == null) return list1;
        ListNode currentNode = null;
        ListNode result = null;
        while (list1 != null && list2 != null) {

            if (list1.val < list2.val) {
                if (currentNode == null) {
                    currentNode = list1;
                    result = currentNode;
                } else {
                    currentNode.next = list1;
                    currentNode = currentNode.next;
                }
                list1 = list1.next;
            } else {
                if (currentNode == null) {
                    currentNode = list2;
                    result = currentNode;
                } else {
                    currentNode.next = list2;
                    currentNode = currentNode.next;
                }
                list2 = list2.next;
            }

        }
        if (list1 == null) {
            currentNode.next = list2;
        }
        if (list2 == null) {
            currentNode.next = list1;
        }
        return result;
    }