debug到手軟的篩法，輸出條件很重要。

 描述：ios 用篩法求［a，b］中的素數。 Find out the prime numbers in [a, b]. less 輸入：spa 2個正整數：a b。 a、b均在1000之內，且a小於等於b。 2 positive integers: a, b. Both a and b are less than or equal 1000 and a is less than or equ

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