Fastest method to define whether a number is a triangular number

Question

A triangular number is the sum of the n natural numbers from 1 to n. What is the fastest method to find whether a given positive integer number is a triangular one?

He

Answer 1

We just need to check if 8*(your integer to be checked)+1 is a perfect square or not!

public Boolean isSquare(int x)
{
    return(Math.sqrt(x)==(int)Math.sqrt(x));    // x will be a perfect square if and only if it's square root is an Integer.
}

}
public Boolean isTriangular(int z)
{
    return(isSquare(8*z+1));
}