数据结构入门-5-链表和递归

链表本身天然就有一个递归的性质

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
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;
    }
}

虚拟头节点解法

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode removeElements(ListNode head, int val) {
        ListNode dumyHead = new ListNode(-1);
        dumyHead.next = head;

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

        }
        return dumyHead.next;
    }
}

本地调试这个算法
ListNode

//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();
    }
}

main

    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);
    }
}

递归

数组求和

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

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

链表的天然递归性

递归解决203删除指定元素

class Solution {
    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;
        }
    }
}

递归函数的微观解读

模拟调用

6--->7--->8--->null

public ListNode removeElements(ListNode head , int val){
	if(head ==  null)
		return null;

	head.next = removeElements(head.next,val);
	return head.val == val ? head.next : head;
}