LintCode：链表操作（合并与反转）

描述：

（1）翻转一个链表

样例

给出一个链表1->2->3->null，这个翻转后的链表为3->2->1->null

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（2）将两个排序链表合并为一个新的排序链表

样例

给出 1->3->8->11->15->null，2->null， 返回 1->2->3->8->11->15->null。

解题思路：

已经定义好ListNode链表类，翻转可以采用栈（stack）来进行读取，也可以采用三个指针来变换，还可以采用递归的方式。

排序就要用到两个指针，头指针和尾指针，尾指针进行数据的指引操作，最后返回头指针即可，也可以采用递归的方式。

代码如下：

import java.util.Stack;

public class 翻转链表 {
    /**
     * Definition for ListNode.
     */
    public class ListNode {
        int val;
        ListNode next;

        ListNode(int val) {
            this.val = val;
            this.next = null;
        }
    }

    /*
     * @param head: n
     *
     * @return: The new head of reversed linked list.
     */
    public ListNode reverse(ListNode head) {
        // write your code here
        if (head == null)
            return null;
        ListNode reverseHead = new ListNode(0);
        ListNode current = reverseHead;
        Stack<ListNode> st = new Stack<ListNode>();//定义栈
        while (head != null) {
            st.push(head);  //压入栈中
            head = head.next;
        }
        while (!st.empty()) {
            current.next = st.pop(); //取出节点
            current = current.next;
        }
        current.next = null;
        return reverseHead.next;
    }

    /**
     * @param head: The head of linked list.
     * @return: The new head of reversed linked list.
     */
    public ListNode reverse2(ListNode head) {
        // write your code here
        ListNode revHead = null;
        ListNode prev = null;
        while(head!=null){
            ListNode pNext = head.next;
            if(pNext == null){
                // 翻转后的头节点,后面是空，结果
                revHead = head;
            }
            // head的下一个节点指向之前要链接的节点
            head.next = prev;
            // 要链接的节点 直线p节点，以供下次链接
            prev = head;
            // head节点更新，指向pNext
            head = pNext;
        }
        return revHead;
    }

    /**
     * 递归
     * @param head
     * @return
     */
    public ListNode reverse3(ListNode head) {
        // write your code here
        if( head ==null || head.next ==null)
            return head;
        ListNode  second = head.next;
        head.next = null;
        ListNode res = reverse(second);
        second.next = head;
        return res;
    }
}

public class 合并两个排序链表 {
    /**
     * Definition for ListNode.
     */
    public class ListNode {
        int val;
        ListNode next;

        ListNode(int val) {
            this.val = val;
            this.next = null;
        }
    }

    /**
     * @param ListNode
     *            l1 is the head of the linked list
     * @param ListNode
     *            l2 is the head of the linked list
     * @return: ListNode head of linked list
     */
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        // write your code here
        if (l1 == null && l2 != null)
            return l2;
        if (l1 != null && l2 == null)
            return l1;
        if (l1 == null && l2 == null)
            return null;
        ListNode head = new ListNode(0); //头指针
        ListNode current = head;         //尾指针
        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                current.next = l1;
                current = current.next;
                l1 = l1.next;
            } else {
                current.next = l2;
                current = current.next;
                l2 = l2.next;
            }
        }

        if (l1 != null)
            current.next = l1;
        if (l2 != null)
            current.next = l2;
        return head.next;
    }

    /**
     * 递归方法
     * @param l1
     * @param l2
     * @return
     */
     public ListNode mergeTwoLists2(ListNode l1, ListNode l2) {
            // write your code here
            if(l1==null && l2!=null)
                return l2;
            if(l1!=null && l2==null)
                return l1;
            if(l1==null && l2==null)
                return null;
            ListNode MergeHead = null;
            if(l1.val < l2.val){
                MergeHead = l1;
                MergeHead.next = mergeTwoLists(l1.next,l2);
            }else{
                MergeHead = l2;
                MergeHead.next = mergeTwoLists(l1,l2.next);
            }
            return MergeHead;

        }

}