I was solving a problem using C++ vector STL. But I was getting some wrong answers for my problem. Then, I started debugging my code. And after that, I noticed some weird behavior by my vector STL.

vector<int> v(n+1);

v.push_back(-3);
for(int i=1;i<=n;i++)
    cin>>v[i];

sort(v.begin(),v.end());

for(int i=0;i<=n;i++)
    cout<<v[i]<<" ";

After giving input like this : 3 5 8 10

I am getting this output : -3 0 3 8 ( why getting 0 on index 1 ? AND where is 10 ? )

And Similarly Let's see Another weird behavior ( if we don't sort the vector v AND still giving same input as of Code 1 ):

vector<int> v(n+1);

v.push_back(-3);
for(int i=1;i<=n;i++)
    cin>>v[i];

for(int i=0;i<=n;i++)
    cout<<v[i]<<" ";

I am getting this output : 0 3 8 10 ( why getting 0 on index 0 ? AND where is -3 ? )

Can someone explain this why I am facing this weird behavior ?