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Java Array Exercises: Count the number of possible triangles from a given unsorted array of positive integers

Java Array: Exercise-44 with Solution

Write a Java program to count the number of possible triangles from a given unsorted array of positive integers.

Note: The triangle inequality states that the sum of the lengths of any two sides of a triangle must be greater than or equal to the length of the third side.

Sample Solution:

Java Code:

import java.util.*;
import java.lang.*;
public class Main
{
   public static void main (String[] args) 
    {  
        int nums[] = {6, 7, 9, 16, 25, 12, 30, 40};
        int n = nums.length;
        System.out.println("Original Array : "+Arrays.toString(nums));  

        // Sort the array elements in non-decreasing order
        Arrays.sort(nums);
        
        // Initialize count of triangles
        int ctr = 0;
 
        for (int i = 0; i < n-2; ++i)
        {
          int x = i + 2;
 
          for (int j = i+1; j < n; ++j)
            {
               while (x < n && nums[i] + nums[j] > nums[x])
               ++x;
                 ctr += x - j - 1;
            }
        }
    System.out.println("Total number of triangles:  " +ctr);
    }
}

Sample Output:

                                                                              
Original Array : [6, 7, 9, 16, 25, 12, 30, 40]
Total number of triangles:  17

Flowchart:

Flowchart: Count the number of possible triangles from a given unsorted array of positive integers

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Java Code Editor:

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Java: Tips of the Day

Array vs ArrayLists:

The main difference between these two is that an Array is of fixed size so once you have created an Array you cannot change it but the ArrayList is not of fixed size. You can create instances of ArrayLists without specifying its size. So if you create such instances of an ArrayList without specifying its size Java will create an instance of an ArrayList of default size.

Once an ArrayList is full it re-sizes itself. In fact, an ArrayList is internally supported by an array. So when an ArrayList is resized it will slow down its performance a bit as the contents of the old Array must be copied to a new Array.

At the same time, it's compulsory to specify the size of an Array directly or indirectly while creating it. And also Arrays can store both primitives and objects while ArrayLists only can store objects.

Ref: https://bit.ly/3o8L2KH