Reverse Linked List II

Source

Problem
Reverse a linked list from position m to n.

Example
Given 1->2->3->4->5->NULL, m = 2 and n = 4, return1->4->3->2->5->NULL.

Note
Given m, n satisfy the following condition: 1 ≤ m ≤ n ≤ length of list.

Challenge
Reverse it in-place and in one-pass

题解

此题在 Reverse Linked List 的基础上加了位置要求，只翻转指定区域的链表。由于链表头节点不确定，祭出我们的 dummy 杀器。此题边界条件处理特别 tricky，需要特别注意。

1. 由于只翻转指定区域，分析受影响的区域为第 m-1 个和第 n+1 个节点
2. 找到第 m 个节点，使用 for 循环 n-m 次，使用上题中的链表翻转方法
3. 处理第 m-1 个和第 n+1 个节点
4. 返回 dummy->next

C++

/**
 * Definition of singly-linked-list:
 *
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *        this->val = val;
 *        this->next = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param head: The head of linked list.
     * @param m: The start position need to reverse.
     * @param n: The end position need to reverse.
     * @return: The new head of partial reversed linked list.
     */
    ListNode *reverseBetween(ListNode *head, int m, int n) {
        if (head == NULL || m > n) {
            return NULL;
        }

        ListNode *dummy = new ListNode(0);
        dummy->next = head;
        ListNode *node = dummy;

        for (int i = 1; i != m; ++i) {
            if (node == NULL) {
                return NULL;
            } else {
                node = node->next;
            }
        }

        ListNode *premNode = node;
        ListNode *mNode = node->next;
        ListNode *nNode = mNode, *postnNode = nNode->next;
        for (int i = m; i != n; ++i) {
            if (postnNode == NULL) {
                return NULL;
            }

            ListNode *temp = postnNode->next;
            postnNode->next = nNode;
            nNode = postnNode;
            postnNode = temp;
        }
        premNode->next = nNode;
        mNode->next = postnNode;

        return dummy->next;
    }
};

Java

/**
 * Definition for ListNode
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param ListNode head is the head of the linked list 
     * @oaram m and n
     * @return: The head of the reversed ListNode
     */
    public ListNode reverseBetween(ListNode head, int m , int n) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;

        // find the mth node
        ListNode premNode = dummy;
        for (int i = 1; i < m; i++) {
            premNode = premNode.next;
        }

        // reverse node between m and n
        ListNode prev = null, curr = premNode.next;
        while (curr != null && (m <= n)) {
            ListNode nextNode = curr.next;
            curr.next = prev;
            prev = curr;
            curr = nextNode;
            m++;
        }

        // join head and tail before m and after n
        premNode.next.next = curr;
        premNode.next = prev;

        return dummy.next;
    }
}

源码分析

1. 处理异常
2. 使用 dummy 辅助节点
3. 找到 premNode，也就是 m 节点之前的一个节点
4. 以 nNode 和 postnNode 进行遍历翻转，注意考虑在遍历到 n 之前 postnNode 可能为空
5. 连接 premNode 和 nNode，premNode->next = nNode;
6. 连接 mNode 和 postnNode，mNode->next = postnNode;

务必注意 node 和 node->next 的区别！！

node 指代节点，而 node->next 指代节点的下一连接。