合并有序链表

牛客网：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

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

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    //有序链表合并
    public ListNode Merge(ListNode list1,ListNode list2) {
        ListNode head = null;
        
        if (list1 == null) {
            if (list2 == null) {
                return head;
            } else {
                head = list2;
                return head;
            }
        } else if (list2 == null){
            head = list1;
            return head;
        } else {
             if (list1.val <= list2.val) {
              head = list1;
              list1 = list1.next;
          } else {
              head = list2;
              list2 = list2.next;
          }
        }
        
        ListNode list = head;
        while ((list1 != null) && (list2 != null)) {
          if (list1.val <= list2.val) {
              list.next = list1;
              list1 = list1.next;
          } else {
              list.next = list2;
              list2 = list2.next;
          }
          list = list.next;
       } 
       
        if (list1 != null) {
            list.next = list1;
        }
        
        if (list2 != null) {
            list.next = list2;
        }
        
        return head;
    }
}