保留小数点后两位 第三位大于零就进一

思想就是先降序 在升序

• (NSString *)multiplying:(double)amount1 and:(double)amount2 afterPoint:(int)position {

  NSDecimalNumberHandler *roundingBehavior = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundUp scale:position raiseOnExactness:NO raiseOnOverflow:NO raiseOnUnderflow:NO raiseOnDivideByZero:NO];

  NSDecimalNumber *decimal1 = [[NSDecimalNumber alloc] initWithDouble:amount1];
  NSDecimalNumber *decimal2 = [[NSDecimalNumber alloc] initWithDouble:amount2];

  NSDecimalNumberHandler *roundingBehavior1 = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundDown scale:3 raiseOnExactness:NO raiseOnOverflow:NO raiseOnUnderflow:NO raiseOnDivideByZero:NO];

  NSDecimalNumber *result = [decimal1 decimalNumberByMultiplyingBy:decimal2 withBehavior:roundingBehavior1];

  NSDecimalNumber *decimal3 = [[NSDecimalNumber alloc] initWithDouble:1.0];

  NSDecimalNumber *result1 = [result decimalNumberByMultiplyingBy:decimal3 withBehavior:roundingBehavior];

  return [result1 stringValue];
  }