Hi!
Why could be time limit on 3 test here:

    private void solve() {
        int N = nextInt();
        String res = "";
        for (int i = 0; i < N; i++) {
            long idx = nextLong();
            double f = Math.sqrt(8*idx - 7);
            if (f - (int)f == 0) {
                res += "1 ";
            } else {
                res += "0 ";
            }
        }
        out.print(res);
    }

May i don't know some fast way to find if integer is perfect square?
I know that on C++ is all right, but want to understand how to make it in java.

I see you have quite a lot of accepted problems in java, so you're not novice. My advice is: generate a big test, then run some kind of profiler or just comment some lines of your code and see when you'll get biggest boost in execution time. After you find problematic line in your code it should become obvious how to fix it.

Tnx for advice) I agree it's the best way... I've just tried to find the fastest one in other people experience :)

----------

found the problem) string concatination is very expensive) for future generations - this works fine:

private void solve() throws IOException {
    int N = nextInt();

    byte[] outArr = new byte[N];
    for (int i = 0; i < N; i++) {
        long idx = nextInt();
        double f = Math.sqrt(8 * idx - 7);
        if (f - (int) f == 0) {
            outArr[i] = 1;
        } else {
            outArr[i] = 0;
        }
    }

    for (int i = 0; i < outArr.length; i++) {
        out.print(outArr[i]);
        out.print(' ');
    }
}

Edited by author 21.02.2012 12:16

Edited by author 21.02.2012 12:17

I'm glad it helped. For future: there is a StringBuilder class in Java, it's in fact a mutable string(contrary to immutable String class). It supports appending characters and strings to itself in linear time, so you could use it in this problem for example. It has other features of course, use Javadocs to find out more!