Today while solving this problem I faced a strange behavior with set<>. I made three identical submissions only with slight changes in compare function to see it's behavior.

My three submission: 1. http://mirror.codeforces.com/contest/802/submission/27695695 verdict : AC compare function :

bool operator < ( const data& b ) const {
    if(pos == b.pos)    {
        return num > b.num;
    }
   return pos < b.pos;
}

1. http://mirror.codeforces.com/contest/802/submission/27695705 verdict : AC compare function :

bool operator < ( const data& b ) const {
    if(pos == b.pos)    {
        return num < b.num;
    }
   return pos < b.pos;
}

1. http://mirror.codeforces.com/contest/802/submission/27695712 verdict : WA compare function :

bool operator < ( const data& b ) const {
   return pos < b.pos;
}

Now, it can be my fault but I always have used third option when I have to compare single variable and class contains multiple variables. Also, choosing this always worked out for me, but it's not the case this time.

Now, I want to know what's causing this and why using third option is wrong? Also, what I should do if I only want to compare single or multiple variables but not all variables?