This is about yesterday's Div2 contest Problem B. For reference, here's my code which got RE on system test 7. Finally, I fixed my code and got AC. Read the blunder I made. I hope you people handle it elegantly the next time you encounter this. Pay attention to the last part of the code which includes the two for loops.

void solve(){
    ll n,a;
    cin>>n;
    vector<ll> o,e;
    rep1(i,2*n){
        cin>>a;
        if(a%2)
            o.pb(i);
        else
            e.pb(i);
    }
    if(e.size()>o.size())
        swap(o,e);
    if(o.size()%2){
        o.pop_back();
        e.pop_back();
    }
    else{
        o.pop_back();
        o.pop_back();   
    }
    for(ll i=0; i<o.size()-1; i+=2){
        wr(o[i], o[i+1]);
    }
    for(ll i=0; i<e.size()-1; i+=2){
        wr(e[i], e[i+1]);
    }
}

The problem here is while outputting the answer, I write the loop's end condition as i < o.size()-1 and i < e.size()-1 . Now, size() returns an unsigned int (In this case, an unsigned 0) and when I subtract 1 from it, like this i < e.size()-1, I get a huge number instead of -1 which I expected I'd get. The loop runs for quite a long time instead of never running at all.

Now, to fix this there are 3 ways you can do so.

1. Typecast the returned value into int like this (int)(e.size()-1).
2. Store this returned value from size method into a variable like this int sz = e.size().
3. As HideBehind mentioned, the simplest way is to write it as i+1 < e.size().

Here's my working code showing both the methods.

void solve(){
    ll n,a;
    cin>>n;
    vector<ll> o,e;
    rep1(i,2*n){
        cin>>a;
        if(a%2)
            o.pb(i);
        else
            e.pb(i);
    }
    if(e.size()>o.size())
        swap(o,e);
    if(o.size()%2){
        o.pop_back();
        e.pop_back();
    }
    else{
        o.pop_back();
        o.pop_back();   
    }
    int sz = o.size();
    for(ll i=0; i<sz-1; i+=2){
        wr(o[i], o[i+1]);
    }
    for(ll i=0; i<(int)e.size()-1; i+=2){
        cout << e[i]<<" "<<e[i+1]<<"\n";
    }
}

I felt like a dumbo after RE on system tests, fixed it, and then thought to inform you all guys (my first blog).

PS: Making a mistake like this deals some serious damage on ratings XD