static int countways ( int a [ ] , int n ) {
  int cnt [ ] = new int [ n ] ;
  int s = 0 ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) {
    s += a [ i ] ;
  }
  if ( s % 3 != 0 ) return 0 ;
  s /= 3 ;
  int ss = 0 ;
  for ( int i = n - 1 ;
  i >= 0 ;
  i -- ) {
    ss += a [ i ] ;
    if ( ss == s ) cnt [ i ] = 1 ;
  }
  for ( int i = n - 2 ;
  i >= 0 ;
  i -- ) cnt [ i ] += cnt [ i + 1 ] ;
  int ans = 0 ;
  ss = 0 ;
  for ( int i = 0 ;
  i + 2 < n ;
  i ++ ) {
    ss += a [ i ] ;
    if ( ss == s ) ans += cnt [ i + 2 ] ;
  }
  return ans ;
}

static int countways ( int a [ ] , int n ) { int cnt [ ] = new int [ n ] ; int s = 0 ; for ( int i = 0 ; i < n ; i ++ ) { s += a [ i ] ; } if ( s % 3 != 0 ) return 0 ; s /= 3 ; int ss = 0 ; for ( int i = n - 1 ; i >= 0 ; i -- ) { ss += a [ i ] ; if ( ss == s ) cnt [ i ] = 1 ; } for ( int i = n - 2 ; i >= 0 ; i -- ) cnt [ i ] += cnt [ i + 1 ] ; int ans = 0 ; ss = 0 ; for ( int i = 0 ; i + 2 < n ; i ++ ) { ss += a [ i ] ; if ( ss == s ) ans += cnt [ i + 2 ] ; } return ans ; }
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