leetCode热题100-27、28题

27、合并两个有序列表

将两个升序链表合并为一个新的 升序 链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。

/**
* 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 l1, ListNode l2) {
       ListNode prehead = new ListNode(-1);

       ListNode prev = prehead;
       while (l1 != null && l2 != null) {
           if (l1.val <= l2.val) {
               prev.next = l1;
               l1 = l1.next;
           }else {
               prev.next = l2;
               l2 = l2.next;
           }
           prev = prev.next;
       }
       prev.next = l1 == null ? l2 : l1;

       return prehead.next;
   }
}

28、删除链表倒数第n个节点

给你一个链表，删除链表的倒数第 n 个结点，并且返回链表的头结点。

/**
 * 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 removeNthFromEnd(ListNode head, int n) {
        ListNode dummy = new ListNode(0,head);
        int length = getLength(head);
        ListNode cur = dummy;
        for (int i = 1; i < length - n + 1;i++) {
            cur = cur.next;
        }
        cur.next = cur.next.next;
        ListNode ans = dummy.next;
        return ans;
    }

    public int getLength(ListNode head) {
        int length = 0;
        while (head != null) {
            ++length;
            head = head.next;
        }
        return length;
    }
}