Leetcode21.合并两个有序链表

question

code

class Solution {
          
   
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
          
   

        ListNode prehead = new ListNode(-1, null);
        ListNode pre = prehead;     //记录新构建链表的尾结点


        while (l1 != null && l2 != null) {
          
   
            if (l1.val < l2.val) {
          
   
                pre.next = l1;
                l1 = l1.next;
            } else {
          
   
                pre.next = l2;
//                l2.next = l2;   //error,枯了
                l2 = l2.next;

            }
            pre = pre.next;
        }

        /*
        最开始，没有写下面两句
        没有处理：一个链表为空，另一个链表未处理完的情况
        处理：直接添加到末尾即可
        结果：[1,1,2,3,4]
        预期结果：[1,1,2,3,4,4]
         */

        if (l1 != null) pre.next = l1;
        if (l2 != null) pre.next = l2;


        return prehead.next;

    }

    class ListNode {
          
   
        int val;
        ListNode next;

        ListNode(int val) {
          
   
            this.val = val;
        }

        public ListNode(int val, ListNode next) {
          
   
            this.val = val;
            this.next = next;
        }

        @Override
        public String toString() {
          
   
            return "ListNode{" +
                    "val=" + val +
                    ", next=" + next +
                    };
        }
    }

}

summary

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