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POGenerate random numbers from 1-100 from a generator of 1-50
 

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  1. POGenerate random numbers from 1-100 from a generator of 1-50
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    <p>in a recent interview i was asked the following question: </p> <blockquote> <p>Print random numbers from 1-100 using the given <code>getrnd50()</code> method which generates the random numbers from 1-50. Each random number should be printed only once and in random order. Use of no other random number generator is allowed and i was not allowed to change the definition of <code>getrnd50()</code>.</p> </blockquote> <p>I came up with the following code which gives the correct output.</p> <pre><code>import java.util.Random; public class Test { public static void main(String[] args) { int[] rs = new int[100]; int count = 0; int k; while (count != 100) { // I decided to simply multiply the result of `getrnd50()` by 2. // But since that would generate only even numbers, k = getrnd50() * 2; // I decided to randomly subtract 1 from the numbers. // Which i accomlished as follows. if (getrnd50() &lt;= 25) // 25 is to half the possibilities. k--; // Every number is to be stored in its own unique location // in the array `rs`, the location is `(number-1)`. // Every time a number is generated it is checked whether it has // already been generated. If not, it is saved in its position, printed and // `count` is incremented. if (rs[k-1] == 0) { rs[k-1] = k; count++; System.out.print(k + " "); } } } // This is the method and i am not supposed to touch it. static int getrnd50() { Random rand = new Random(); return (1 + rand.nextInt(50)); } } </code></pre> <p>While it was accepted in that round, in the next round the interviewer tells me that <code>getrnd50()</code> is a costly method and even in best case scenario i have to call it twice for every number generated. i.e. 200 times for 1-100. In worst case scenario it would be infinity and tens of thousand in average case. He asks me to optimize the code so as to <strong>significantly</strong> improve the average case.</p> <p>He gave me a hint when i expressed my inability to do it, he said: </p> <blockquote> <p>To consider the number of numbers generated while generating a new number. For ex. if <code>count</code> becomes 99 i don't have to call <code>getrnd50()</code> I can simply find the remaining number and print it.</p> </blockquote> <p>While i understood his drift i had no idea how it would help me, so obviously i got rejected. Now i am curious to know the answer. Help me! Thanx in advance!</p> <p><strong>Note:</strong> if anyone is feeling lazy to to write a lengthy code just point out the numer generation part, the rest is easy. Also we are not bound to follow the hint.</p>
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    1. USSurender Thakran
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    1. COWhat they're asking for is not, in fact, a random number generator if they want each number exactly once. It's a shuffler - you could just as easily populate an array with numbers 1-100 and then repeatedly swap numbers at random indices.
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    2. CO@Alex so why would they provide `getrnd50()` because we are not even using it in your solution i think.
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    3. CO@SurenderThakran Sure we are, we're selecting random indices.
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