Java solution for “Longest Increasing Subsequence”

Thursday, December 10th, 2020

The Problem

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 which the subsequence is as long as possible. This subsequence is not necessarily contiguous, or unique.

https://en.wikipedia.org/wiki/Longest_increasing_subsequence

My solution

This is what I came up with (albeit in a short timeframe); it’s not the best or most efficient!

Main.java

package com.lstierneyltd.examples;

public class Main {
    public static void main(String[] args) throws Exception {
        final LongestIncreasingSubsequenceSolver solver =
                new LongestIncreasingSubsequenceSolver();


        solver.init(new int[]{10, 22, 9, 33, 21, 50, 41, 60});
        solver.solve(); // 5

        // Van der Corput sequence
        solver.init(new int[]{0, 8, 4, 12, 2, 10, 6, 14, 1, 9, 5, 13, 3, 11, 7, 15});
        solver.solve(); // 6

    }
}

LongestIncreasingSubsequenceSolver.java

package com.lstierneyltd.examples;

import java.util.ArrayList;
import java.util.List;
public class LongestIncreasingSubsequenceSolver {
    private int inputLength;
    private List<Integer> sequence;
    private List<Integer> input;

    public void init(final int[] inputArray) {
        inputLength = inputArray.length;
        input = new ArrayList<>(inputLength);
        sequence = new ArrayList<>(inputLength);

        int i;
        for (i = 0; i < inputLength; i++) {
            input.add(inputArray[i]);
        }

        for (i = 0; i < inputLength; i++) {
            sequence.add(i, 1);
        }
    }

    public void solve() {
        int i, j, max = 0;

        for (i = 1; i < inputLength; i++) {
            for (j = 0; j < i; j++) {
                if (input.get(i) > input.get(j) && sequence.get(i) < sequence.get(j) + 1) {
                    sequence.set(i, sequence.get(j) + 1);
                }
            }
        }
        for (i = 0; i < inputLength; i++) {
            if (max < sequence.get(i)) {
                max = sequence.get(i);
            }
        }
        System.out.println("The Longest Subsequence length was: " + max);
    }
}

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Tags: java
Posted in: Interview Questions