function to calculate the coefficients of ( x - 1 ) ^ n - ( x ^ n - 1 ) with the help of Pascal 's triangle .
void coef ( int n ) { c [ 0 ] = 1 ; for ( int i = 0 ; i < n ; c [ 0 ] = - c [ 0 ] , i ++ ) { c [ 1 + i ] = 1 ; for ( int j = i ; j > 0 ; j -- ) c [ j ] = c [ j - 1 ] - c [ j ] ; } }