劍指offer 15：反轉鏈表

題目描述

輸入一個鏈表，反轉鏈表後，輸出新鏈表的表頭。

法一：迭代法

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head == null)
            return null;
        ListNode pre = null;
        ListNode next = null;
      
        while(head != null){
            next = head.next;
            head.next = pre;
            pre = head;
            head = next;
        }
        return pre;
    }
}

法二：遞歸法

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head == null ||head.next == null)
            return head;
        ListNode next = head.next;
        head.next = null;
        ListNode newHead = ReverseList(next);
        next.next = head;
        return newHead;
    }
}