劍指 offer：反轉鏈表

題目描述

輸入一個鏈表，反轉鏈表後，輸出鏈表的所有元素。

思想：遍歷鏈表，改用頭插法翻轉鏈表。

/*
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 newHead = null;//這個一定要爲空，不能爲head，因爲要保證翻轉後鏈表最後一個元素的next＝null
        ListNode pre = head;
        ListNode p;
        while(pre!=null){
           p = pre.next;
           pre.next = newHead;
           newHead = pre;
           pre = p;
        }
         return newHead;
     }
}

</pre><p></p><p style="font-size:14px; line-height:1.6; margin-bottom:20px; color:rgb(51,51,51); font-family:arial,STHeiti,'Microsoft YaHei',宋體">思想：利用棧，先將鏈表從頭到尾放入棧中，然後從新建立新的鏈表，並輸出。</p><p style="font-size:14px; line-height:1.6; margin-bottom:20px; color:rgb(51,51,51); font-family:arial,STHeiti,'Microsoft YaHei',宋體"></p><pre name="code" class="java">import java.util.Stack;

public class Solution {
    public ListNode ReverseList(ListNode head) {
           if(head == null){
                return null;
            }
            Stack<Integer> stack = new Stack<Integer>();
            ListNode p = head;
            while(p!=null){
                stack.push(p.val);
                p = p.next;
            }
            ListNode first = new ListNode(stack.pop());
            ListNode h = first;
            System.out.print(" "+ h.val);
            while(!stack.empty()){
                ListNode node = new ListNode(stack.pop());
                h.next = node;
                h = h.next;
                System.out.print(" "+ h.val);
             }
        	return first;
     }
}