LeetCode--链表

19. Remove Nth Node From End of List 

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        if (head == null || head.next == null) {
            return null;
        }
        ListNode start = new ListNode(0); //学会保存链表的头
        start.next = head;
        ListNode slow = start;
        ListNode fast = start;
        for (int i = 0; i <= n; i++) {
            fast = fast.next;
        }
        while (fast != null) {
            slow = slow.next;
            fast = fast.next;
        }
        slow.next = slow.next.next;
        return start.next;
    }
}

 21. Merge Two Sorted Lists

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1 == null) {
            return l2;
        }
        if (l2 == null) {
            return l1;
        }
        ListNode newHead = null; //采用递归调用的思想
        if (l1.val < l2.val) {
            newHead = l1;
            newHead.next = mergeTwoLists(l1.next, l2);
        } else {
            newHead = l2;
            newHead.next = mergeTwoLists(l1, l2.next);
        }
        return newHead;
    }
}