翻转链表

剑指 Offer 24. 反转链表

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        if(head==null || head.next==null) 
            return head;
        ListNode pre = head;
        ListNode now = pre.next;
        head.next =null;
        while(now!=null) {
            ListNode t = now.next;
            now.next = pre;
            pre = now;
            now = t;
        }
        return pre;
    }
}

删除倒数第n个节点

1. 对于可能删除头节点的情况，先创建一个虚拟节点指向头节点，使头节点作为第一个而不是第零个出现

148. 排序链表

恶心透了

/**
 * 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 sortList(ListNode head) {
        if(head==null||head.next==null)
            return head;
        int n = 0;
        ListNode now = head;
        while(now!=null) {
            now = now.next;
            n++;
        }
        ListNode dummy = new ListNode(0,head);
        for(int sub = 1; sub < n;sub=sub*2) {
            ListNode pre = dummy;
            ListNode curr = pre.next;
            while(curr!=null) {
                ListNode head1 = curr;
                for(int i = 1; i < sub && curr.next!=null ; i++) {
                    curr = curr.next;
                }
                ListNode head2 = curr.next;
                curr.next = null;
                curr = head2;
                for(int i = 1; i < sub && curr!=null && curr.next!=null ;i++) {
                    curr = curr.next;       
                }
                ListNode next = null;
                if(curr!=null) {
                    next = curr.next;
                    curr.next = null;
                }
                ListNode merged = merge(head1,head2);
                pre.next = merged;
                while(pre.next!=null) 
                    pre = pre.next;
                curr = next;
            }
        }
        return dummy.next;
    }
    public ListNode merge(ListNode head1, ListNode head2) {
        ListNode dummyHead = new ListNode(0);
        ListNode temp = dummyHead, temp1 = head1, temp2 = head2;
        while (temp1 != null && temp2 != null) {
            if (temp1.val <= temp2.val) {
                temp.next = temp1;
                temp1 = temp1.next;
            } else {
                temp.next = temp2;
                temp2 = temp2.next;
            }
            temp = temp.next;
        }
        if (temp1 != null) {
            temp.next = temp1;
        } else if (temp2 != null) {
            temp.next = temp2;
        }
        return dummyHead.next;
    }
}