public static int sum ( int a [ ] , int n ) {
  Map < Integer , Integer > cnt = new HashMap < Integer , Integer > ( ) ;
  int ans = 0 , pre_sum = 0 ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) {
    ans += ( i * a [ i ] ) - pre_sum ;
    pre_sum += a [ i ] ;
    if ( cnt . containsKey ( a [ i ] - 1 ) ) ans -= cnt . get ( a [ i ] - 1 ) ;
    if ( cnt . containsKey ( a [ i ] + 1 ) ) ans += cnt . get ( a [ i ] + 1 ) ;
    if ( cnt . containsKey ( a [ i ] ) ) {
      cnt . put ( a [ i ] , cnt . get ( a [ i ] ) + 1 ) ;
    }
    else {
      cnt . put ( a [ i ] , 1 ) ;
    }
  }
  return ans ;
}

public static int sum ( int a [ ] , int n ) { Map < Integer , Integer > cnt = new HashMap < Integer , Integer > ( ) ; int ans = 0 , pre_sum = 0 ; for ( int i = 0 ; i < n ; i ++ ) { ans += ( i * a [ i ] ) - pre_sum ; pre_sum += a [ i ] ; if ( cnt . containsKey ( a [ i ] - 1 ) ) ans -= cnt . get ( a [ i ] - 1 ) ; if ( cnt . containsKey ( a [ i ] + 1 ) ) ans += cnt . get ( a [ i ] + 1 ) ; if ( cnt . containsKey ( a [ i ] ) ) { cnt . put ( a [ i ] , cnt . get ( a [ i ] ) + 1 ) ; } else { cnt . put ( a [ i ] , 1 ) ; } } return ans ; }
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