代码随想录算法训练营第三天 | 链表概念、移除链表元素、设计链表、反转

链表概念

移除链表元素

体会虚拟头结点的便利

/*
 * @lc app=leetcode.cn id=203 lang=java
 *
 * [203] 移除链表元素
 */

// @lc code=start
/**
 * 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 removeElements(ListNode head, int val) {
        ListNode a = new ListNode(-1, head);
        ListNode b = a;
        while (b.next != null && b != null) {
            if (b.next.val == val) {
                b.next = b.next.next;
            } else {
                b = b.next;
            }
        }
        return a.next;
    }
}
// @lc code=end

设计链表

理解链表的基本操作，注意边界值问题，理清思路

/*
 * @lc app=leetcode.cn id=707 lang=java
 *
 * [707] 设计链表
 */

// @lc code=start
class MyLinkedList {
    class ListNode {
        int val;
        ListNode next;
        ListNode(int val) { this.val = val; }
    }

    private ListNode head;  // 头节点
    private int length;

    public MyLinkedList() {
        head = null;
        length = 0;
    }

    public int get(int index) {
        if (index < 0 || index >= length) return -1;
        ListNode cur = head;
        for (int i = 0; i < index; i++) cur = cur.next;
        return cur.val;
    }

    public void addAtHead(int val) {
        ListNode newNode = new ListNode(val);
        newNode.next = head;
        head = newNode;
        length++;
    }

    public void addAtTail(int val) {
        if (head == null) {
            addAtHead(val);
            return;
        }
        ListNode cur = head;
        while (cur.next != null) cur = cur.next;
        cur.next = new ListNode(val);
        length++;
    }

    public void addAtIndex(int index, int val) {
        if (index > length) return;
        if (index == 0) {
            addAtHead(val);
            return;
        }
        // 找到 index 的前一个节点
        ListNode prev = head;
        for (int i = 0; i < index - 1; i++) prev = prev.next;
        ListNode newNode = new ListNode(val);
        newNode.next = prev.next;
        prev.next = newNode;
        length++;
    }

    public void deleteAtIndex(int index) {
        if (index < 0 || index >= length) return;
        if (index == 0) {
            head = head.next;
        } else {
            ListNode prev = head;
            for (int i = 0; i < index - 1; i++) prev = prev.next;
            prev.next = prev.next.next;
        }
        length--;
    }
}

/**
 * Your MyLinkedList object will be instantiated and called as such:
 * MyLinkedList obj = new MyLinkedList();
 * int param_1 = obj.get(index);
 * obj.addAtHead(val);
 * obj.addAtTail(val);
 * obj.addAtIndex(index,val);
 * obj.deleteAtIndex(index);
 */
// @lc code=end

反转链表

头插法解决

/*
 * @lc app=leetcode.cn id=206 lang=java
 *
 * [206] 反转链表
 */

// @lc code=start
/**
 * 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 reverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode temp = new ListNode();
        ListNode result = new ListNode();
        ListNode a = new ListNode();
        temp.next = head;
        while (temp.next != null) {
            temp = temp.next;
            a = new ListNode(temp.val);
            a.next = result.next;
            result.next = a;
        }
        return result.next;
    }
}
// @lc code=end