I created a set of structure and sorted was manually. See the code...

class edge    {
public:
    int u, v, w;//variables on set
    edge()  {}
    edge(int a, int b,int c)  {
        u = a;
        v = b;
        w = c;
    }
    bool operator < ( const edge& z ) const {
        return w < z.w;//compare as your wish 
    }
};
set<edge> e;

Then tried to insert some element as u v w

1 2 10

1 3 8

3 2 3

1 4 3

1 3 6

2 1 2

But 1 4 3 didn't added. Then i used multiset and it worked . why?? we know set doesn't contain duplicate element. But there are no duplicate.