玩转数据结构：第5章 链表和递归

链表和递归

---

5-1 Leetcode中和链表相关的问题 

Java类的递归，包含的成员变量有该类本身。

ListNode

//Definition for singly-linked list.
public class ListNode {

    public int val;
    public ListNode next;

    public ListNode(int x) {
        val = x;
    }
}

/// Leetcode 203. Remove Linked List Elements
/// https://leetcode.com/problems/remove-linked-list-elements/description/

 Solution

/// Leetcode 203. Remove Linked List Elements
/// https://leetcode.com/problems/remove-linked-list-elements/description/

class Solution {

    public ListNode removeElements(ListNode head, int val) {

        while(head != null && head.val == val){
            ListNode delNode = head;
            head = head.next;
            delNode.next = null;
        }

        if(head == null)
            return head;

        ListNode prev = head;
        while(prev.next != null){
            if(prev.next.val == val) {
                ListNode delNode = prev.next;
                prev.next = delNode.next;
                delNode.next = null;
            }
            else
                prev = prev.next;
        }

        return head;
    }
}

 Solution2

/// Leetcode 203. Remove Linked List Elements
/// https://leetcode.com/problems/remove-linked-list-elements/description/

class Solution2 {

    public ListNode removeElements(ListNode head, int val) {

        while(head != null && head.val == val)
            head = head.next;

        if(head == null)
            return head;

        ListNode prev = head;
        while(prev.next != null){
            if(prev.next.val == val)
                prev.next = prev.next.next;
            else
                prev = prev.next;
        }

        return head;
    }
}

 Solution3

/// Leetcode 203. Remove Linked List Elements
/// https://leetcode.com/problems/remove-linked-list-elements/description/

class Solution3 {

    public ListNode removeElements(ListNode head, int val) {

        ListNode dummyHead = new ListNode(-1);
        dummyHead.next = head;

        ListNode prev = dummyHead;
        while(prev.next != null){
            if(prev.next.val == val)
                prev.next = prev.next.next;
            else
                prev = prev.next;
        }

        return dummyHead.next;
    }
}

5-2 测试自己的Leetcode链表代码 

//Definition for singly-linked list.
public class ListNode {

    public int val;
    public ListNode next;

    public ListNode(int x) {
        val = x;
    }

    // 链表节点的构造函数
    // 使用arr为参数，创建一个链表，当前的ListNode为链表头结点
    public ListNode(int[] arr){

        if(arr == null || arr.length == 0)
            throw new IllegalArgumentException("arr can not be empty");

        this.val = arr[0];
        ListNode cur = this;
        for(int i = 1 ; i < arr.length ; i ++){
            cur.next = new ListNode(arr[i]);
            cur = cur.next;
        }
    }

    // 以当前节点为头结点的链表信息字符串
    @Override
    public String toString(){

        StringBuilder s = new StringBuilder();
        ListNode cur = this;
        while(cur != null){
            s.append(cur.val + "->");
            cur = cur.next;
        }
        s.append("NULL");
        return s.toString();
    }
}

class Solution3

class Solution3 {

    public ListNode removeElements(ListNode head, int val) {

        ListNode dummyHead = new ListNode(-1);
        dummyHead.next = head;

        ListNode prev = dummyHead;
        while(prev.next != null){
            if(prev.next.val == val)
                prev.next = prev.next.next;
            else
                prev = prev.next;
        }

        return dummyHead.next;
    }

    public static void main(String[] args) {

        int[] nums = {1, 2, 6, 3, 4, 5, 6};
        ListNode head = new ListNode(nums);
        System.out.println(head);

        ListNode res = (new Solution3()).removeElements(head, 6);
        System.out.println(res);
    }
}

5-3 递归基础与递归的宏观语意 

 本质上，将原来的问题，转化为更小的同一问题。

 递归函数就是一个函数，完成一个功能。

public class Sum {

    public static int sum(int[] arr){
        return sum(arr, 0);
    }

    // 计算arr[l...n)这个区间内所有数字的和
    private static int sum(int[] arr, int l){
        if(l == arr.length)
            return 0;
        return arr[l] + sum(arr, l + 1);
    }

    public static void main(String[] args) {

        int[] nums = {1, 2, 3, 4, 5, 6, 7, 8};
        System.out.println(sum(nums));
    }
}

 5-4 链表的天然递归结构性质

class Solution4 {

    public ListNode removeElements(ListNode head, int val) {

        if(head == null)
            return head;

        ListNode res = removeElements(head.next, val);
        if(head.val == val)
            return res;
        else{
            head.next = res;
            return head;
        }
    }

    public static void main(String[] args) {

        int[] nums = {1, 2, 6, 3, 4, 5, 6};
        ListNode head = new ListNode(nums);
        System.out.println(head);

        ListNode res = (new Solution4()).removeElements(head, 6);
        System.out.println(res);
    }
}

 5-5 递归运行的机制：递归的微观解读