Count triplets.cpp

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
int countTriplets(int arr[], int N)
{
	// Sort the array
	sort(arr, arr + N);

	// Stores count of triplets
	int l, r, i, j, ans = 0;

	// Iterate over the range [1, N]
	for (i = 0; i < N; i++) {
		// Iterate over the range [i+1, N]
		for (j = i + 1; j < N; j++) {
			// Required difference
			int x = arr[j] - arr[i];

			// Find lower bound of arr[j] + x
			// and upper bound of arr[j] + 2*x
			l = lower_bound(arr, arr + N, arr[j] + x) - arr;
			r = upper_bound(arr, arr + N, arr[j] + 2 * x)
				- arr;

			// Adjust the indexing of arr[j]+2*r
			if (r == N || arr[r] != arr[j] + 2 * x)
				r -= 1;

			// From l to r, count number of
			// triplets by using arr[i] and arr[j]
			ans += r - l + 1;
		}
	}
	// return main ans
	return ans;
}
// Driver code
int main()
{
	int arr[] = { 1, 2, 3 };
	int N = sizeof(arr) / sizeof(arr[0]);
	int ans = countTriplets(arr, N);
	cout << ans;

	return 0;
}