leetcode — reverse-linked-list-ii

/**
* Source : https://oj.leetcode.com/problems/reverse-linked-list-ii/ *
*
* Reverse a linked list from position m to n. Do it in-place and in one-pass.
*
* For example:
* Given 1->2->3->4->5->NULL, m = 2 and n = 4,
*
* return 1->4->3->2->5->NULL.
*
* Note:
* Given m, n satisfy the following condition:
* 1 ≤ m ≤ n ≤ length of list.
*/
public class ReverseLinkedList2 {

/**
*
* 反转单向链表指定范围内的元素
*
* 需要考虑第一个元素是否被翻转
*
* @param head
* @param m
* @param n
* @return
*/
public Node reverse (Node head, int m, int n) {
if (head == null) {
return head;
}
Node dummy = new Node();
dummy.next = head;
int pos = 1;
Node unreverseListLast = dummy;
Node reverseListLast = null;
Node cur = head;
Node next = null;
Node pre = dummy;
while (cur != null && pos <= n) {
next = cur.next;
if (pos == m) {
unreverseListLast = pre;
reverseListLast = cur;
} else if (pos > m) {
// 在制定范围内，反转
cur.next = pre;
}
pre = cur;
cur = next;
pos++;
}

// 反转完指定范围内的元素，将反转部分和未反转部分连接起来
unreverseListLast.next = pre;
reverseListLast.next = cur;

return dummy.next;
}

private static class Node {
int value;
Node next;

@Override
public String toString() {
return "Node{" +
"value=" + value +
", next=" + (next == null ? "" : next.value) +
'}';
}
}

private static void print (Node node) {
while (node != null) {
System.out.println(node);
node = node.next;
}
System.out.println();
}

public Node createList (int[] arr) {
if (arr.length == 0) {
return null;
}
Node head = new Node();
head.value = arr[0];
Node pointer = head;
for (int i = 1; i < arr.length; i++) {
Node node = new Node();
node.value = arr[i];
pointer.next = node;
pointer = pointer.next;
}
return head;
}

public static void main(String[] args) {
ReverseLinkedList2 reverseLinkedList2 = new ReverseLinkedList2();
print(reverseLinkedList2.reverse(reverseLinkedList2.createList(new int[]{1,2,3,4,5}), 2, 4));
print(reverseLinkedList2.reverse(reverseLinkedList2.createList(new int[]{1,2,3,4,5}), 2, 2));
print(reverseLinkedList2.reverse(reverseLinkedList2.createList(new int[]{1,2,3,4,5}), 2, 5));
print(reverseLinkedList2.reverse(reverseLinkedList2.createList(new int[]{1,2,3,4,5}), 1, 5));
print(reverseLinkedList2.reverse(reverseLinkedList2.createList(new int[]{}), 1, 1));
}

}