Codechef Puzzle: NUMGAME

So I encounter this interesting problem NUMGAME at Codechef. This is in the list of easy problems.
At first I was lost after looking the problem and could not think of how to solve it but when I came to thinking about it, it seems that it is quite easy.

Hints: (Obvious but important)

1) An odd number is always formed by the multiplication of two odd numbers.
2) If you substract an odd number from an odd number then it gives you an even number.

GCD Recursive Method

The greatest common divisor (g.c.d.) of two nonnegative integers is the largest integer that divides evenly into both. In the third century B.C., the Greek mathematician Euclid discovered that the greatest common divisor of x and y can always be computed as follows:

If x is evenly divisible by y, then y is the greatest common divisor. Otherwise, the greatest common divisor of x and y is always equal to the greatest common divisor of y and the remainder of x divided by y


private static int gcdmethod(int i, int j) {

if (i%j==0) {
return j;
}
return gcdmethod(j,i%j);
}

}

Boundary Fill Algorithm

Just recorded a video on Boundary Fill Algorithm:

Code : http://theunacademy.blogspot.com/2011/01/boundary-full-algorithm.html

Scan Line Polygon Fill Algorithm

This video covers the concepts of Scan Line Polygon Filling. Takes into account the special cases and ways to resolve them.

Reference:
Computer Graphics, Hearn, Baker