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Simplified and a bit more optimized solution #154

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Simplified and a bit more optimized solution #154

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29 changes: 5 additions & 24 deletions epi_judge_java_solutions/epi/RandomSubset.java
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Original file line number Diff line number Diff line change
Expand Up @@ -17,32 +17,13 @@ public class RandomSubset {

// Returns a random k-sized subset of {0, 1, ..., n - 1}.
public static List<Integer> randomSubset(int n, int k) {

Map<Integer, Integer> changedElements = new HashMap<>();
Random randIdxGen = new Random();
for (int i = 0; i < k; ++i) {
// Generate random int in [i, n - 1].
int randIdx = i + randIdxGen.nextInt(n - i);
Integer ptr1 = changedElements.get(randIdx);
Integer ptr2 = changedElements.get(i);
if (ptr1 == null && ptr2 == null) {
changedElements.put(randIdx, i);
changedElements.put(i, randIdx);
} else if (ptr1 == null && ptr2 != null) {
changedElements.put(randIdx, ptr2);
changedElements.put(i, randIdx);
} else if (ptr1 != null && ptr2 == null) {
changedElements.put(i, ptr1);
changedElements.put(randIdx, i);
} else {
changedElements.put(i, ptr1);
changedElements.put(randIdx, ptr2);
}
}

List<Integer> result = new ArrayList<>(k);
HashMap<Integer,Integer> changedElements = new HashMap<>();
Random randIdxGen = new Random();
for (int i = 0; i < k; ++i) {
result.add(changedElements.get(i));
int randIdx = i + randIdxGen.nextInt(n-i);
result.add(changedElements.getOrDefault(randIdx,randIdx));
changedElements.put(randIdx,changedElements.getOrDefault(i,i));
}
return result;
}
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