Program exit for unknown reason (0xC0000005)

Question

I'm practicing Longest Increasing Sequence problem and I was using this code:

int lis(vector<int>& a)
{
    int s = a.size();
    vector<int> prev(s);
    vector< vector<int> > pile;
    int m = -1; // lis size
    for(int i=0; i<s; i++)
    {
        // initialize piles
        vector<int> vt;
        pile.push_back(vt);
    }

    pile[0].push_back(0); // a[0] must go to the first pile
    prev[0] = -1;

    for(int i=1; i<s; i++)
    {
        cout << "i: " << i << endl;
        for(int j=0; j<s; j++)
        {
            cout << "j: " << j << endl;
            if(pile[j].size() == 0)
            {
                printf("pile %d is empty, push %d (%d)\n", j, i, a[i]);

                pile[j].push_back(i);
                if(j == 0)
                {
                    prev[i] = -1;
                }
                else
                {
                    prev[i] = pile[j-1].back();
                }

                m = max(j, m);
                break;
            }
            else if(a[pile[j].back()] < a[i])
            {
                printf("go to pile %d\n", j+1);
                continue;
            }
            else
            {
                printf("push %d (%d) to pile %d\n", i, a[i], j);

                pile[j].push_back(i);
                if(j == 0)
                {
                    prev[i] = -1;
                }
                else
                {
                    prev[i] = pile[j-1].back();
                }

                m = max(j, m);
                break;
            }
        }
    }

    // print out the sequences
    int ind = pile[m][0];
    vector<int> res;
    while(ind != -1)
    {
        res.push_back(a[ind]);
        ind = prev[ind];
    }
    for(int i=res.size()-1; i>=0; i--)
    {
        cout << res[i] << " ";
    }

    return m+1; // return lis size
}

Which was, working fine. Then, I wanted to simplify the code and came up with this:

int lis(vector<int>& a)
{
    /* same */

    for(int i=1; i<s; i++)
    {
        cout << "i: " << i << endl;
        for(int j=0; j<s; j++)
        {
            cout << "j: " << j << endl;
            if(a[pile[j].back()] > a[i] || pile[j].size() == 0)
            {
                printf("push %d to pile %d\n", a[i], j);

                pile[j].push_back(i);
                prev[i] = (j == 0) ? -1 : pile[j-1].back();                    
                m = max(j, m);

                break;
            }
            printf("go to pile %d\n", j+1);
        }
    }

    /* same */
}

With this code, when it goes to the next pile (j increases), the program suddenly exits. (Process returned 0xC0000005)
This couldn't be buffer overflow or segmentation fault because every vector/array is declared correctly and the index isn't exceeding the bound.
This also couldn't be memory leak or dangling pointer because I didn't use any manual allocation.
I couldn't sort this out!

What did I do wrong?

Answer 1

To begin with, in the following statement:

if(a[pile[j].back()] > a[i] || pile[j].size() == 0)

I believe that you should check the second part of the expression first.

In other words, check pile[j].size() == 0 first, in order to avoid calling pile[j].back() if true.