016 合并两个排序的链表

1.题目

　　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。

2.分析

　　

3.程序

package first;

import java.util.ArrayList;

public class Merge {
    public static void main(String[] args){
        //第一个链表
        ListNode head = new ListNode();
        ListNode second = new ListNode();
        ListNode third = new ListNode();
        ListNode forth = new ListNode();
        head.next = second;
        second.next = third;
        third.next = forth;
        head.val = 1;
        second.val = 2;
        third.val = 3;
        forth.val = 4;
        //第二个链表
        ListNode head2 = new ListNode();
        ListNode second2 = new ListNode();
        ListNode third2 = new ListNode();
        ListNode forth2 = new ListNode();
        head2.next = second2;
        second2.next = third2;
        third2.next = forth2;
        head2.val = 3;
        second2.val = 4;
        third2.val = 5;
        forth2.val = 6;
        //打印出来
        ListNode listNode=Merge2(head,head2);
        ArrayList list=printListFromTailToHead(listNode);
        printInfo(list);

    }

    /**
     * 非递归
     */
    static ArrayList<Integer> arrayList=new ArrayList<Integer>();
    public static ListNode Merge(ListNode list1,ListNode list2) {
        ListNode newHead = new ListNode(-1);
        ListNode current = newHead;
        while (list1 != null && list2 != null) {
            if (list1.val < list2.val) {
                current.next = list1;
                list1 = list1.next;
            } else {
                current.next = list2;
                list2 = list2.next;
            }
            current = current.next;
        }
        if (list1 != null) current.next = list1;
        if (list2 != null) current.next = list2;
        return newHead.next;
    }

    /**
     * 递归
     */
    public static ListNode Merge2(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;
        }
    }

    public static ArrayList<Integer> printListFromTailToHead(ListNode listNode) {
        if(listNode!=null){
            printListFromTailToHead(listNode.next);
            arrayList.add(listNode.val);
        }
        return arrayList;
    }

    public static void printInfo(ArrayList list) {
        for(int i=0;i<list.size();i++) {
            System.out.print(list.get(i)+" ");
        }
        System.out.println("");
    }

}