题解 | #链表内指定区间反转#

先找出翻转区间前一个节点left和后一个节点right，然后用常规的翻转链表方法把翻转区间翻转。最后拼接起来。注意翻转区间的最后要设null打断，防止无限循环

/*
 * function ListNode(x){
 *   this.val = x;
 *   this.next = null;
 * }
 */

/**
  * 
  * @param head ListNode类 
  * @param m int整型 
  * @param n int整型 
  * @return ListNode类
  */
function reverse(head) {
    let pre = null;
    let cur = head;
    let next;
    while(cur) {
        next = cur.next;
        cur.next = pre;
        
        pre = cur;
        cur = next;
    }
    return [pre, head];
}
function reverseBetween( head ,  m ,  n ) {
    // 增加一个origin方便返回最终结果 origin.next
    // 因为head也有可能被翻转了
    const origin = { next: head };
    head = origin;
    let left, right;
    let i = 0;
    for(; i < m - 1; i++) {
        head = head.next;
    }
    left = head;
    for(; i < n; i++) {
        head = head.next;
    }
    right = head.next;
    head.next = null;
    const [ start, end ] = reverse(left.next);
    left.next = start;
    end.next = right;
    return  origin.next
}
module.exports = {
    reverseBetween : reverseBetween
};