Ambiguous Permutations Problem

[ChosenTorture]

Consider the following question:
http://www.spoj.com/problems/PERMUT2/

The code that I've written is:

#include<iostream>
#include<vector>

using std::cin;
using std::cout;
using std::vector;

int main()
{
    int num;
    vector<int> arr1, arr2;
    while(cin >> num)
    {
        arr1.push_back(num);
    }
    auto n = arr1.size();
    for(int i=0; i!=n; ++i)
    {
        arr2[arr1[i]-1] = (i+1);
    }
    for(int i=0; i!=n; ++i)
    {
        if(arr1[i]!=arr2[i])
        {
            cout << "Not Ambiguous\n";
            break;
        }
        else
            continue;
        cout << "Ambiguous\n";
    }
    return 0;
}

I don't understand what's wrong with this code. It just keeps crashing after taking inputs properly. Can anyone help me?

[MiiNiPaa]

arr2[arr1[i]-1]
arr2 is empty and doent have any elements. You are accessing out of bounds (or trying to dereference a null pointer depending on realization).
If you not sure if you can accidently try to access out of bounds, use at() member functions, at leas it wil give you a hint what the problem is: arr2.at(arr1[i]-1)

[ChosenTorture]

Wouldn't the same problem persist even after I use the at() function? How can I correct this issue?

[MiiNiPaa]

It will, but you will see what the problem is next time. In your case you should rethink your program algorithm.

Actually you can do arr2.reserve(arr1.size()) but your code still isn't very good.

[ChosenTorture]

your code still isn't very good.

Any Suggestions for Improvement?