static int countPairs ( int arr [ ] , int n ) {
  HashMap < Integer , Integer > map = new HashMap < Integer , Integer > ( ) ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) map . put ( arr [ i ] - i , 0 ) ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) map . put ( arr [ i ] - i , map . get ( arr [ i ] - i ) + 1 ) ;
  int res = 0 ;
  for ( int x : map . values ( ) ) {
    int cnt = x ;
    res += ( ( cnt * ( cnt - 1 ) ) / 2 ) ;
  }
  return res ;
}

static int countPairs ( int arr [ ] , int n ) { HashMap < Integer , Integer > map = new HashMap < Integer , Integer > ( ) ; for ( int i = 0 ; i < n ; i ++ ) map . put ( arr [ i ] - i , 0 ) ; for ( int i = 0 ; i < n ; i ++ ) map . put ( arr [ i ] - i , map . get ( arr [ i ] - i ) + 1 ) ; int res = 0 ; for ( int x : map . values ( ) ) { int cnt = x ; res += ( ( cnt * ( cnt - 1 ) ) / 2 ) ; } return res ; }
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