nck [ i ] [ j ] = number of ways to choose j elements form i elements , no order
let nck = new Array ( MAXN ) ; for ( let i = 0 ; i < MAXN ; i ++ ) { nck [ i ] = new Array ( MAXN ) ; for ( let j = 0 ; j < MAXN ; j ++ ) nck [ i ] [ j ] = 0 ; }