Find all ranges of consecutive numbers from Array

Given a sorted array arr[] consisting of N integers without any duplicates, the task is to find the ranges of consecutive numbers from that array.

Examples:

Input: arr[] = {1, 2, 3, 6, 7}
Output: 1->3, 6->7
Explanation:
There are two ranges of consecutive number from that array.
Range 1 = 1 -> 3
Range 2 = 6 -> 7

Input: arr[] = {-1, 0, 1, 2, 5, 6, 8}
Output: -1->2, 5->6, 8
Explanation:
There are three ranges of consecutive number from that array.
Range 1 = -1 -> 2
Range 2 = 5 -> 6
Range 3 = 8

Approach: The idea is to traverse the array from the initial position and for every element in the array, check the difference between the current element and the previous element.



  • If the difference between the current element and the previous element is 1 then we just increment the length variable. We use the length variable to build the range “A -> B”. Since only the range is required, we don’t need to store all the elements between A and B. We just only need to know the length of this range.
  • If the difference between the current element and the previous element is doesn’t equal to 1, we build the range between the first element of the range and the current previous element as the last range.

Below is the implementation of the above approach:

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// Java program to find the ranges of
// consecutive numbers from array
  
import java.util.*;
  
class GFG {
  
    // Function to find consecutive ranges
    static List<String>
    consecutiveRanges(int[] a)
    {
        int length = 1;
        List<String> list
            = new ArrayList<String>();
  
        // If the array is empty,
        // return the list
        if (a.length == 0) {
            return list;
        }
  
        // Traverse the array from first position
        for (int i = 1; i <= a.length; i++) {
  
            // Check the difference between the
            // current and the previous elements
            // If the difference doesn't equal to 1
            // just increment the length variable.
            if (i == a.length
                || a[i] - a[i - 1] != 1) {
  
                // If the range contains
                // only one element.
                // add it into the list.
                if (length == 1) {
                    list.add(
                        String.valueOf(a[i - length]));
                }
                else {
  
                    // Build the range between the first
                    // element of the range and the
                    // current previous element as the
                    // last range.
                    list.add(a[i - length]
                             + " -> " + a[i - 1]);
                }
  
                // After finding the first range
                // initialize the length by 1 to
                // build the next range.
                length = 1;
            }
            else {
                length++;
            }
        }
  
        return list;
    }
  
    // Driver Code.
    public static void main(String args[])
    {
  
        // Test Case 1:
        int[] arr1 = { 1, 2, 3, 6, 7 };
        System.out.print(consecutiveRanges(arr1));
        System.out.println();
  
        // Test Case 2:
        int[] arr2 = { -1, 0, 1, 2, 5, 6, 8 };
        System.out.print(consecutiveRanges(arr2));
        System.out.println();
  
        // Test Case 3:
        int[] arr3 = { -1, 3, 4, 5, 20, 21, 25 };
        System.out.print(consecutiveRanges(arr3));
    }
}

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Output:

[1 -> 3, 6 -> 7]
[-1 -> 2, 5 -> 6, 8]
[-1, 3 -> 5, 20 -> 21, 25]

Time Complexity: O(N), where N is the length of the array.

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