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Commit 7a7549d

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Closes #91 Bug fix
1 parent e6ff7a8 commit 7a7549d

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2 files changed

+36
-16
lines changed

2 files changed

+36
-16
lines changed
Lines changed: 35 additions & 15 deletions
Original file line numberDiff line numberDiff line change
@@ -1,9 +1,5 @@
11
package com.jwetherell.algorithms.sequence;
22

3-
import com.jwetherell.algorithms.search.LowerBound;
4-
5-
import java.util.Arrays;
6-
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/**
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* In computer science, the longest increasing subsequence problem is to find a subsequence of a given sequence in which the subsequence's elements are in sorted order, lowest to highest, and in
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* which the subsequence is as long as possible. This subsequence is not necessarily contiguous, or unique.
@@ -20,19 +16,43 @@ private LongestIncreasingSubsequence() { }
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/**
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* Longest increasing subsequence solved using dynamic programming.
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*/
23-
public static int[] getLongestIncreasingSubsequence(int[] sequence) {
24-
final int[] resultSequence = new int[sequence.length];
19+
public static int[] getLongestIncreasingSubsequence(int[] X) {
20+
final int[] P = new int[X.length];
21+
final int[] M = new int[X.length+1];
22+
int L = 0;
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for (int i=0; i<X.length; i++) {
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// Binary search for the largest positive j ≤ L such that X[M[j]] < X[i]
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int lo = 1;
26+
int hi = L;
27+
while (lo <= hi) {
28+
final int mid = (int) Math.ceil((lo + hi) / 2);
29+
if (X[M[mid]] < X[i])
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lo = mid+1;
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else
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hi = mid-1;
33+
}
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35+
// After searching, lo is 1 greater than the length of the longest prefix of X[i]
36+
final int newL = lo;
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// The predecessor of X[i] is the last index of the subsequence of length newL-1
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P[i] = M[newL-1];
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M[newL] = i;
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42+
if (newL > L) {
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// If we found a subsequence longer than any we've found yet, update L
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L = newL;
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}
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}
2547

26-
int resultLength = 0;
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for (int i = 0; i < sequence.length; ++i) {
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// try to find the best place for new element in sorted result
29-
final int pos = LowerBound.lowerBound(resultSequence, resultLength, sequence[i]); //O(log n)
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// if the best place is at the end increase result length
31-
if (pos >= resultLength)
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resultLength++;
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resultSequence[pos] = sequence[i];
48+
// Reconstruct the longest increasing subsequence
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final int[] S = new int[L];
50+
int k = M[L];
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for (int i=L-1; i>=0; i--) {
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S[i] = X[k];
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k = P[k];
3454
}
3555

36-
return Arrays.copyOfRange(resultSequence, 0, resultLength);
56+
return S;
3757
}
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}

‎test/com/jwetherell/algorithms/sequence/test/Sequences.java‎

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -141,7 +141,7 @@ public void testLongestIncreasingSubsequence() {
141141
sequencesLis.add(new int[]{1, 2, 3});
142142

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sequences.add(new int[]{0, 8, 4, 12, 2, 10, 6, 14, 1, 9, 5, 13, 3, 11, 7, 15});
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sequencesLis.add(new int[]{0, 1, 3, 7, 11, 15});
144+
sequencesLis.add(new int[]{0, 2, 6, 9, 11, 15});
145145

146146
assertTrue("Longest increasing subsequence error. Sequences size=" + sequences.size() + " SequencesList size:" + sequencesLis.size(), sequences.size() == sequencesLis.size());
147147

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