LeetCode【206.反转链表】

题目描述

反转一个单链表。

示例：

• 输入： 1->2->3->4->5->NULL

• 输出：5->4->3->2->1->NULL

思路1

采用迭代的方式，改变各个结点的指针的方向。其重点在于首结点应该指向NULL。

代码1

class Solution {
    public ListNode reverseList(ListNode head) {
   // 改变指针方向,Iterative
        ListNode dummpyRoot =new  ListNode(-1);
        ListNode node = head;
        while(node != null) {
            ListNode next = node.next;
            node.next = dummpyRoot.next;
            dummpyRoot.next = node;
            node = next;
        }
        return dummpyRoot.next;
      }
 }

复杂度分析：

时间复杂度：$O(n)$
空间复杂度：$O(1)$

思路2

采用递归的方式
（1）我们每次使用$head.next.next=head;head.next=null;$改变指针的方向。
（2）我们把迭代的方式改为递归（两个参数）的方式，每次递归相当于每次的迭代。

代码：

(1)

class Solution {
    public ListNode reverseList(ListNode head) {
 //Recursive
        if(head == null || head.next == null)
            return head;
        ListNode node = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return node;
        }
  }

(2)

class Solution {
    public ListNode reverseList(ListNode head) {
	    return reverseListInt(head, null);
    }
     private ListNode reverseListInt(ListNode head, ListNode newHead) {
        if (head == null)
            return newHead;
        ListNode next = head.next;
        head.next = newHead;
        return reverseListInt(next, head);
    }
  }

复杂度分析：

时间复杂度：$O(n)$
空间复杂度：$O(n)$

完整代码

package cn.zzuli.zcs8;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

/**
 * Created by 张超帅 on 2018/9/18.
 */
public class leetcode206 {


    public static int[] stringToIntegerArray(String input) {
        input = input.trim();
        input = input.substring(1,input.length() - 1);
        if(input.length() == 0) {
            return new int[0];
        }
        String[] parts = input.split(",");
        int[] output = new int[parts.length];
        for (int index = 0; index < parts.length; index ++) {
            String part = parts[index].trim();
            output[index] = Integer.parseInt(part);
        }
        return output;
    }

    public static ListNode stringToListNode(String intput) {
        int[] nodeValues = stringToIntegerArray(intput);
        ListNode dummyRoot = new ListNode(0);
        ListNode ptr = dummyRoot;
        for (int item : nodeValues) {
            ptr.next = new ListNode(item);
            ptr = ptr.next;
        }
        return dummyRoot.next;
    }
    public static String listNodeToString(ListNode head) {
        if(head == null) {
            return "[]";
        }
        String result = "";
        ListNode node = head;
        while (node != null) {
            result += Integer.toString(node.val) +", ";
            node = node.next;
        }
        return "[" + result.substring(0,result.length() - 2) +"]";
    }
    public static void main(String[] args) throws IOException{
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        String line ;
        while((line = in.readLine()) != null) {
            ListNode head = stringToListNode(line);
            ListNode ret = new Solution().reverseList(head);
            String out =listNodeToString(ret);
            System.out.println(out);
        }

    }
}
class ListNode {
    int val ;
    ListNode next;
    ListNode(int x) {val = x;}
}
class Solution {
    public ListNode reverseList(ListNode head) {
        /* 改变指针方向,Iterative
        ListNode dummpyRoot =new  ListNode(-1);
        ListNode node = head;
        while(node != null) {
            ListNode next = node.next;
            node.next = dummpyRoot.next;
            dummpyRoot.next = node;
            node = next;
        }
        return dummpyRoot.next;
        */
        //Recursive
        /*if(head == null || head.next == null)
            return head;
        ListNode node = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return node;
        */
        return reverseListInt(head, null);
    }
    private ListNode reverseListInt(ListNode head, ListNode newHead) {
        if (head == null)
            return newHead;
        ListNode next = head.next;
        head.next = newHead;
        return reverseListInt(next, head);
    }
}