《剑指offer》第15题——反转链表

题目：

输入一个链表，反转链表后，输出新链表的表头。

思路：

利用头插法，或者利用栈

Java

利用头插法：

package nowcoder;

public class S15_ReverseList {
    public ListNode reverseList(ListNode head){
        //利用头插法
        ListNode pre = head;
        ListNode p, q;
        if (head == null)
            return null;
        p = pre.next;
        while (p !=null){
            q = p.next;
            pre.next = null;
            p.next = pre;
            pre = p;
            p = q;
        }
        return p;
    }
    public static void main(String[] args){
        S15_ReverseList s15 = new S15_ReverseList();
        S14_FindKthToTail s14 = new S14_FindKthToTail();
        int[] array = {1,2,3,4,5,6,7,8,10};
        ListNode head = s14.arrayToList(array, 0);
        ListNode result = s15.reverseList(head);
        if (result == null)
            System.out.print("null");
        else
            System.out.print(result.val);
    }
}

利用栈

package nowcoder;

import java.util.Stack;

public class S15_ReverseList {
    public ListNode reverseList(ListNode head){
        //利用栈
        Stack<ListNode> stack = new Stack<ListNode>();
        ListNode p = head;
        //最后一个节点不进栈
        while (p.next != null){
            stack.push(p);
            p = p.next;
        }
        ListNode q = p; //最后一个节点
        while (!stack.isEmpty()){
            q.next = stack.pop();
            q = q.next;
        }
        return p;
    }
    public static void main(String[] args){
        S15_ReverseList s15 = new S15_ReverseList();
        S14_FindKthToTail s14 = new S14_FindKthToTail();
        int[] array = {1,2,3,4,5,6,7,8,10};
        ListNode head = s14.arrayToList(array, 0);
        ListNode result = s15.reverseList(head);
        if (result == null)
            System.out.print("null");
        else
            System.out.print(result.val);
    }
}

Python

from S14_FindKthToTail14 import FindKthToTail
class ReverseList:
    def reverseList(self, pHead):
        if not pHead or not pHead.next:
            return pHead
        p = pHead.next
        pHead.next = None
        while p:
            p1 = p.next
            p.next = pHead
            pHead = p
            p = p1
        return pHead

if __name__ == '__main__':
    test = ReverseList()
    test14 = FindKthToTail()
    array = [1, 2, 3, 4, 5, 6, 7, 8, 9]
    head = test14.arrayToList(array, 0)
    print(test.reverseList(head).val)