The challenge

A format for expressing an ordered list of integers is to use a comma separated list of either

  • individual integers
  • or a range of integers denoted by the starting integer separated from the end integer in the range by a dash, ‘-‘. The range includes all integers in the interval including both endpoints. It is not considered a range unless it spans at least 3 numbers. For example “12,13,15-17”

Complete the solution so that it takes a list of integers in increasing order and returns a correctly formatted string in the range format.

Example:

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Solution.rangeExtraction(new int[] {-6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20})
// returns "-6,-3-1,3-5,7-11,14,15,17-20"

The solution in Java code

Option 1:

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class Solution {
    public static String rangeExtraction(int[] arr) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < arr.length; i++) {
            sb.append(arr[i]);
            int j = i;
            while (j < arr.length - 1 && arr[j] + 1 == arr[j + 1]) j++;
            if (i + 1 < j) {
                i = j;
                sb.append("-");
                sb.append(arr[i]);
            }
            sb.append(",");
        }
        sb.setLength(sb.length() - 1);
        return sb.toString();
    }
}

Option 2:

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class Solution {
    static String rangeExtraction(final int[] array) {
        final StringBuilder sb = new StringBuilder();
        for (int i = 0; i < array.length; i++) {
            int rangeStart = array[i];
            sb.append(rangeStart);
            for (int j = i + 1; j <= array.length; j++) {
                if (j == array.length || array[j] != rangeStart + (j - i)) {
                    if (j - i >= 3) {
                        sb.append('-').append(array[j - 1]);
                        i = j - 1;
                    }
                    sb.append(',');
                    break;
                }
            }
        }
        return sb.deleteCharAt(sb.length() - 1).toString();
    }
}

Option 3:

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import java.util.*;
class Solution {
    public static String rangeExtraction(int[] arr) {
        String res = "";
        boolean isStart = true;
        for (int i = 0; i < arr.length - 1; i++) {
            boolean isChecked = (arr[i] + 1 != arr[i + 1]);
            res += isChecked ? arr[i] + "," : isStart ? arr[i] + "~" : "~";
            isStart = isChecked;
        }
        return res.replaceAll("([~]{2,})","-").replace("~", ",") + arr[arr.length - 1];
    }
}

Test cases to validate our solution

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import org.junit.Test;
import static org.junit.Assert.assertEquals;
import org.junit.runners.JUnit4;

public class SolutionTest {
    @Test
    public void test_BasicTests() {
        assertEquals("-6,-3-1,3-5,7-11,14,15,17-20", Solution.rangeExtraction(new int[] {-6,-3,-2,-1,0,1,3,4,5,7,8,9,10,11,14,15,17,18,19,20}));
				
        assertEquals("-3--1,2,10,15,16,18-20", Solution.rangeExtraction(new int[] {-3,-2,-1,2,10,15,16,18,19,20}));
    }
}