/*
 * @lc app=leetcode id=92 lang=java
 *
 * [92] Reverse Linked List II
 */

// @lc code=start
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    /**
     * 链表逆序：头插法
     * @param head
     * @param left
     * @param right
     * @return
     */
    public ListNode reverseBetween(ListNode head, int left, int right) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode prev, frm;
        prev = frm = dummy;
        for(int i = 0; i<left-1; i++) {
            prev = prev.next;
        }
        for(int i = 0; i < left; i++) {
            frm = frm.next;
        }
        ListNode cur;
        for(int i = 0; i<right-left; i++) {
            cur = frm.next;
            frm.next = cur.next;
            cur.next = prev.next;
            prev.next = cur;
        }
        return dummy.next;
    }
}
// @lc code=end