Question

I am working on an algorithm, and I need to be able to pass in a List and see if there are four numbers in a row at any point in the list.

I have been struggling with an easy way to do this… Here is the basic idea.. I would like the fourNumbersInARow() method to return true:

import java.util.ArrayList;
import java.util.List;
import java.util.Random;


public class Numbers {

    /**
     * @param args
     */
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<Integer>();
        for(int i = 0; i<10; i++){
            numbers.add((new Random().nextInt()));
        }
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);

        System.out.println(fourNumbersInARow());

    }

    private static boolean fourNumbersInARow() {


    }

}

Answer 1

Here’s an implementation in Java.

static boolean fourNumbersInARow(List<Integer> list) {
    int last = 0xFACADE;  // can be any number
    int count = 0;        // important!
    for (int i : list) {
        if (i == last + 1) {
            if (++count == 4) return true;
        } else {
            count = 1;
        }
        last = i;
    }
    return false;
}

Unlike others, this resets the count of numbers in a row to 1 when the sequence is broken (because a number on its own is 1 number in a row). This allows for easier treatment of the first iteration where technically there is no previous number.