Getting runtime error

Question

Hi. I'm getting a runtime error for this with Last executed input: {5}, 1, 1

The code is not great, but i've tested it on my machine and it appears to work. Moreover, because of the very first if condition, I have no idea how that test case could fail :S

public ListNode reverseBetween(ListNode head, int m, int n) {
    if(head == null || m == n) return head;
    ListNode head_ = head; // save old head

    for(int i = 1; i < m-1; i++) head = head.next;

    ListNode start = head;
    ListNode revEnd = m == 1 ? head : head.next;
    ListNode prev = revEnd;
    head = prev;

    for(int i = m; i <= n; i++){
        ListNode next = head.next;
        head.next = prev;
        prev = head;
        head = next;
    }

    // Thread appropriately
    revEnd.next = head;

    if(m == 1) head_ = prev; // if we need to replace the head
    else       start.next = prev;

    return head_;
}

Anyone see any problems? I'm surprised since it shouldn't get any farther than the first if statement. Thanks.

Comments

That's curious, indeed. I tried your code and it crashed with a runtime error; then I tried mine and it went through without problem. I wonder if your problem is not with another test case. In particular, are you sure you don't get a NullPointerException for the instruction head.next = prev when i==n==length of the list?

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Yeah, it's probably crashing on some other test case. But I still can't figure it out, tested it on all 1 <= m <= n <= length of list for a list of size 1, 2, 3, 4, 5 and I don't get any errors.

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Sorry, I have fixed this issue. Your code is AC now :)

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It was an OJ problem? What was it exactly, I'm curious now :)

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I'm also very curious!