21.合并两个有序链表.md

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
            ListNode flag = new ListNode(-1);
            ListNode p = flag;
            ListNode p1 = list1;
            ListNode p2 = list2;
            //To check the list is not null
            while(p1 != null &&  p2 != null)
            {
                if(p1.val<p2.val)
                {
                    p.next = p1;
                    p1 = p1.next;
                }
                else
                {
                    p.next = p2;
                    p2 = p2.next;
                }
                p = p.next;
            }
     if (p1 != null) {
        p.next = p1;
    }
    
    if (p2 != null) {
        p.next = p2;
    }
    
    return flag.next;
    }
}