剑指Offer -15 - 反转链表 - C++/Java

题目描述

输入一个链表，反转链表后，输出新链表的表头。

解题思路

头插法

具体代码

• C++

/*
struct ListNode {
	int val;
	struct ListNode *next;
	ListNode(int x) :
			val(x), next(NULL) {
	}
};*/

class Solution
{
public:
    ListNode *ReverseList(ListNode *pHead)
    {
        ListNode *q, *pre;
        q = pHead;  //记录后继结点，防止断链
        pre = NULL; //记录头结点位置(新链表表头)
        while (pHead)
        {
            q = pHead->next; //q记录当前结点的后继结点
            pHead->next = pre; //摘下当前结点插入新链表的表头
            pre = pHead; //pre始终指向新链表的表头
            pHead = q; //工作指针pHead后移
        }
        return pre;
    }
};

• Java

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

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode ReverseList(ListNode head) {
        ListNode q = null; //记录工作指针head的后继结点，防止断链
        ListNode pre = null; //记录前驱结点（新新链表的表头）
        while(head != null){
            q = head.next;
            head.next = pre;
            pre = head;
            head = q;
        }
        return pre;
    }
}