From 36d7a05184900b9cd6492d66b18d306ddc89a8b1 Mon Sep 17 00:00:00 2001
From: yogeshwalecha <62990948+yogeshwalecha@users.noreply.github.com>
Date: Sat, 2 Oct 2021 16:28:00 +0530
Subject: [PATCH] Create CountTriplets.cpp

---
 DSA/Cpp/CountTriplets.cpp | 55 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)
 create mode 100644 DSA/Cpp/CountTriplets.cpp

diff --git a/DSA/Cpp/CountTriplets.cpp b/DSA/Cpp/CountTriplets.cpp
new file mode 100644
index 0000000..edf4287
--- /dev/null
+++ b/DSA/Cpp/CountTriplets.cpp
@@ -0,0 +1,55 @@
+
+// C++ program to count Triplets such that at
+// least one of the numbers can be written
+// as sum of the other two
+#include<bits/stdc++.h>
+using namespace std;
+ 
+    // Function to count the number of ways to choose the triples..
+    // 
+    int countWays(int arr[], int n)
+    {
+        // compute the max value in the array and create frequency array of size max_val + 1.
+         
+        // We can also use HashMap to store frequencies. We have used an array to keep remaining code simple.
+        int max_val = 0;
+        for (int i = 0; i < n; i++)
+            max_val = max(max_val, arr[i]);
+        int freq[max_val + 1]={0};
+        for (int i = 0; i < n; i++)
+            freq[arr[i]]++;
+ 
+        int ans = 0; // stores the number of ways
+ 
+        // Case 1: 0, 0, 0
+        ans += freq[0] * (freq[0] - 1) * (freq[0] - 2) / 6;
+ 
+        // Case 2: 0, x, x
+        for (int i = 1; i <= max_val; i++)
+            ans += freq[0] * freq[i] * (freq[i] - 1) / 2;
+ 
+        // Case 3: x, x, 2*x
+        for (int i = 1; 2 * i <= max_val; i++)
+            ans += freq[i] * (freq[i] - 1) / 2 * freq[2 * i];
+ 
+        // Case 4: x, y, x + y
+        // iterate through all pairs (x, y)
+        for (int i = 1; i <= max_val; i++) {
+            for (int j = i + 1; i + j <= max_val; j++)
+                ans += freq[i] * freq[j] * freq[i + j];
+        }
+ 
+        return ans;
+    }
+ 
+    int main()
+    {
+      int n;
+      cin>>n;
+      int arr[n];
+      for(int i=0;i<n;i++){
+      cin>>arr[i];
+      }
+        cout<<(countWays(arr, n));
+        return 0;
+    }