97 lines
2.8 KiB
Java
97 lines
2.8 KiB
Java
import java.util.ArrayList;
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import java.util.HashMap;
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import java.util.List;
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import java.util.Map;
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class Hailstone {
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public static List<Long> getHailstoneSequence(long n) {
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if (n <= 0)
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throw new IllegalArgumentException("Invalid starting sequence number");
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List<Long> list = new ArrayList<Long>();
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list.add(Long.valueOf(n));
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while (n != 1) {
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if ((n & 1) == 0)
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n = n / 2;
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else
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n = 3 * n + 1;
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list.add(Long.valueOf(n));
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}
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return list;
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}
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public static void main(String[] args) {
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List<Long> sequence27 = getHailstoneSequence(27);
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System.out.println("Sequence for 27 has " + sequence27.size() + " elements: " + sequence27);
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long MAX = 100000;
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// Simple way
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{
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long highestNumber = 1;
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int highestCount = 1;
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for (long i = 2; i < MAX; i++) {
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int count = getHailstoneSequence(i).size();
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if (count > highestCount) {
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highestCount = count;
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highestNumber = i;
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}
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}
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System.out.println("Method 1, number " + highestNumber + " has the longest sequence, with a length of " + highestCount);
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}
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// More memory efficient way
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{
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long highestNumber = 1;
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int highestCount = 1;
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for (long i = 2; i < MAX; i++) {
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int count = 1;
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long n = i;
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while (n != 1) {
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if ((n & 1) == 0)
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n = n / 2;
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else
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n = 3 * n + 1;
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count++;
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}
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if (count > highestCount) {
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highestCount = count;
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highestNumber = i;
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}
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}
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System.out.println("Method 2, number " + highestNumber + " has the longest sequence, with a length of " + highestCount);
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}
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// Efficient for analyzing all sequences
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{
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long highestNumber = 1;
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long highestCount = 1;
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Map<Long, Integer> sequenceMap = new HashMap<Long, Integer>();
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sequenceMap.put(Long.valueOf(1), Integer.valueOf(1));
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List<Long> currentList = new ArrayList<Long>();
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for (long i = 2; i < MAX; i++) {
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currentList.clear();
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Long n = Long.valueOf(i);
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Integer count = null;
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while ((count = sequenceMap.get(n)) == null) {
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currentList.add(n);
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long nValue = n.longValue();
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if ((nValue & 1) == 0)
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n = Long.valueOf(nValue / 2);
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else
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n = Long.valueOf(3 * nValue + 1);
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}
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int curCount = count.intValue();
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for (int j = currentList.size() - 1; j >= 0; j--)
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sequenceMap.put(currentList.get(j), Integer.valueOf(++curCount));
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if (curCount > highestCount) {
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highestCount = curCount;
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highestNumber = i;
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}
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}
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System.out.println("Method 3, number " + highestNumber + " has the longest sequence, with a length of " + highestCount);
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}
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return;
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}
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}
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