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。

AC代码

• 递归版本

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public 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;
        }
    }
}

• 非递归版本

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode Merge(ListNode list1,ListNode list2) {
        if(list1 == null)
            return list2;
        if(list2 == null)
            return list1;
        ListNode mergeHead = null;
        ListNode current = null;
        while(list1 != null && list2 != null){
            if(list1.val <= list2.val){
                if(mergeHead == null){
                    mergeHead = current = list1;
                }else{
                    current.next = list1;
                    current = current.next;
                }
                list1 = list1.next;
            }else{
                if(mergeHead == null){
                    mergeHead = current = list2;
                }else{
                    current.next = list2;
                    current = current.next;
                }
                list2 = list2.next;
            }
        }
        if(list1 == null)
            current.next = list2;
        if(list2 == null)
            current.next = list1;
        return mergeHead;
    }
}