2023.3.24

13：用双链表结构实现双端队列

// 使用双链表实现双端队列
public class DoubleLinkedListToDeque<V> {

    public static class Node<V> {
        V value;
        Node<V> next;
        Node<V> last;

        public Node(V value) {
            this.value = value;
        }
    }

    Node<V> head;
    Node<V> tail;
    int usedSize;

    public DoubleLinkedListToDeque() {
        head = null;
        tail = null;
        usedSize = 0;
    }

    public boolean isEmpty() {
        return this.usedSize == 0;
    }

    public int size() {
        return this.usedSize;
    }

    public void pushHead(V value) {
        Node<V> cur = new Node<>(value);
        if(head == null) {
            tail = cur;
            head = cur;
        }else {
            cur.next = head;
            head.last = cur;
            head = cur;
        }
        usedSize++;
    }

    public void pushTail(V value) {
        Node<V> cur = new Node<>(value);
        if(head == null) {
            tail = cur;
            head = cur;
        }else {
            tail.next = cur;
            cur.last = tail;
            tail = cur;
        }
        usedSize++;
    }

    public V popHead() {
        if(this.head == null) {
            return null;
        }
        V cur = this.head.value;
        if(this.head == this.tail) {
            this.head = null;
            this.tail = null;
        }else {
            head = head.next;
            // 别忘了这一步
            head.last = null;
        }
        usedSize--;
        return cur;
    }

    public V popTail() {
        if(this.head == null) {
            return null;
        }
        V cur = this.tail.value;
        if(this.head == this.tail) {
            this.head = null;
            this.tail = null;
        }else {
            tail = tail.last;
            // 别忘了这一步
            tail.next = null;
        }
        usedSize--;
        return cur;
    }

    public V peekHead() {
        return this.isEmpty() ? null : this.head.value;
    }

    public V peekTail() {
        return this.isEmpty() ? null : this.tail.value;
    }
}

14：K个节点的组内逆序调整

给定一个单链表的头节点head，和一个正数k

实现k个节点的小组内部逆序，如果最后一组不够k个就不调整

例子:

调整前：1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8，k = 3

调整后：3 -> 2 -> 1 -> 6 -> 5 -> 4 -> 7 -> 8

// K 个一组翻转链表

public class ReverseNodesInKGroup {

    public static class ListNode {
        int val;
        ListNode next;

        ListNode() {
        }

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

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

    public ListNode reverseKGroup(ListNode head, int k) {
        ListNode start = head;
        ListNode end = getKGroupEnd(start , k);
        if(end == null) {
            return head;
        }
        // 说明有第一段
        head = end;
        reverse(start, end);
        while(start.next != null) {
            ListNode lastEnd = start;
            start = lastEnd.next;
            end = getKGroupEnd(start,k);
            if(end == null) {
                return head;
            }
            // 说明有第二段
            reverse(start, end);
            lastEnd.next = end;
        }
        return head;
    }

    // 得到一段区域的末尾节点
    public static ListNode getKGroupEnd(ListNode start, int k) {
        for (int i = 1; start != null && i < k; i++) {
            start = start.next;
        }
        return start;
    }

    // 把这一段进行反转
    public static void reverse(ListNode start, ListNode end) {
        ListNode pre = null;
        ListNode cur = start;
        ListNode next = null;
        while(pre != end) {
            next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        start.next = cur;
    }
}