I use unsigned long long and STL vector with very simple algorithm: store x1, x2, x3 in the vector, then find the minimal diffenrence and push it in the vector (repeat the process until minimal difference is 0). I got AC.
I suppose that the tests are not strong enough. I wonder whether there is any test whose output is very large since I don's believe vector can store too many elements.
P/S: I'm a beginner so I'm very glad to study from all of you :-)!