链表

从一道leecode体面中先看
题目：删除链表中等于给定值val的所有元素
示例：
给定：1->2->3->8->4->56->8，val=8
返回：1->2->3->4->56
coding

package arithmetic.recursive;

public class ListNode {

    public int val;
    public ListNode next;

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

package arithmetic.recursive;

public 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 null;
        }

        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;


    }
}

下面我们自己写一个创建链表

package arithmetic.recursive;
    
    public class ListNode {
    
        public int val;
        public ListNode next;
    
        public ListNode(int val) {
            this.val = val;
        }
    
        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=0;i<arr.length;i++){
                cur.next = new ListNode(arr[i]);
                cur = cur.next;
            }
        }
    
        @Override
        public String toString() {
            StringBuilder res = new StringBuilder();
            ListNode cur = this;
            while (cur!=null){
                res.append(cur.val + " ->");
                cur = cur.next;
            }
            res.append("NULL");
            return res.toString();
        }
    }

package arithmetic.recursive;

public class Solution {
    public ListNode removeElements(ListNode head, int val) {

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

        if (head == null) {
            return null;
        }

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

        return head;


    }

    public static void main(String[] args) {
        int[] nums = {1, 3, 45, 6, 8, 2, 5, 78, 8};
        ListNode head = new ListNode(nums);
        System.out.println(head);
        System.out.println("--------");

        ListNode res = (new Solution()).removeElements(head, 8);
        System.out.println(res);


    }
}

递归

本质上，将原来的问题，转化为更小的问题
举例：数组求和链表与递归
coding

package arithmetic.recursive;

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, 9, 10};
        System.out.println(sum(nums));
    }

}

链表和递归的关系

链表的天然递归性

链表与递归用递归解决上面删除元素的问题
`package arithmetic.recursive;

import com.sun.org.apache.bcel.internal.generic.RET;

//使用虚拟头结点
public class Solution3 {
public ListNode removeElements(ListNode head, int val) {

    if (head == null) {
        return null;
    }

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

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

}

public static void main(String[] args) {
    int[] nums = {1, 3, 45, 6, 8, 2, 5, 78, 10, 9, 8};
    ListNode head = new ListNode(nums);
    System.out.println(head);
    System.out.println("--------");

    ListNode res = (new Solution()).removeElements(head, 8);
    System.out.println(res);

}