Count triplets such that sum of any two number is equal to third | Set 2

13 Aug, 2021
Given an array of distinct positive integers arr[] of length N, the task is to count all the triplets such that sum of two elements equals the third element.
Examples:  
Input: arr[] = {1, 5, 3, 2} Output: 2 Explanation: In the given array, there are two such triplets such that sum of the two numbers is equal to the third number, those are – (1, 2, 3), (3, 2, 5)Input: arr[] = {3, 2, 7} Output: 0 Explanation: In the given array there are no such triplets such that sum of two numbers is equal to the third number.  
Approach: The idea is to create a frequency array of the numbers which are present in the array and then check for each pair of the element that the sum of the pair elements is present in the array or not with the help of frequency array in O(1) time.Algorithm:  
Declare a frequency array to store the frequency of the numbers.
Iterate over the elements of the array and increment the count of that number in the frequency array.
Run two loops to choose two different indexes of the matrix and check if the sum of the elements at those indices has a frequency more than 0 in the frequency array.
If frequency of the sum is greater than 0:
    Increment the count of the triplets.
Note: We have assumed in the program that the value of array elements lies in the range [1, 100].Below is the implementation of the above approach: 
C++
Java
Python 3
C#
Javascript
// C++ implementation to count the// triplets such that the sum of the// two numbers is equal to third number #include <bits/stdc++.h>using namespace std; // Function to find the count of the// triplets such that sum of two// numbers is equal to the third numberint countTriplets(int arr[], int n){    int freq[100] = {0};         // Loop to count the frequency    for (int i=0; i < n; i++){        freq[arr[i]]++;    }    int count = 0;         // Loop to count for triplets    for(int i = 0;i < n; i++){        for(int j = i+1; j < n; j++){            if(freq[arr[i] + arr[j]]){                count++;            }        }    }    return count;} // Driver Codeint main(){    int n = 4;    int arr[] = {1, 5, 3, 2};         // Function Call    cout << countTriplets(arr, n);    return 0;}
// Java implementation to count the// triplets such that the sum of the// two numbers is equal to third numberclass GFG{ // Function to find the count of the// triplets such that sum of two// numbers is equal to the third numberstatic int countTriplets(int arr[], int n){    int []freq = new int[100];         // Loop to count the frequency    for (int i = 0; i < n; i++){        freq[arr[i]]++;    }    int count = 0;         // Loop to count for triplets    for(int i = 0; i < n; i++){        for(int j = i + 1; j < n; j++){            if(freq[arr[i] + arr[j]] > 0){                count++;            }        }    }    return count;} // Driver Codepublic static void main(String[] args){    int n = 4;    int arr[] = {1, 5, 3, 2};         // Function Call    System.out.print(countTriplets(arr, n));}} // This code is contributed by Rajput-Ji
# Python 3 implementation to count the# triplets such that the sum of the# two numbers is equal to third number # Function to find the count of the# triplets such that sum of two# numbers is equal to the third numberdef countTriplets(arr, n):    freq = [0 for i in range(100)]         # Loop to count the frequency    for i in range(n):        freq[arr[i]] += 1    count = 0         # Loop to count for triplets    for i in range(n):        for j in range(i + 1, n, 1):            if(freq[arr[i] + arr[j]]):                count += 1    return count # Driver Codeif __name__ == '__main__':    n = 4    arr = [1, 5, 3, 2]         # Function Call    print(countTriplets(arr, n)) # This code is contributed by Surendra_Gangwar
// C# implementation to count the// triplets such that the sum of the// two numbers is equal to third numberusing System; class GFG{ // Function to find the count of the// triplets such that sum of two// numbers is equal to the third numberstatic int countTriplets(int []arr, int n){    int []freq = new int[100];         // Loop to count the frequency    for (int i = 0; i < n; i++){        freq[arr[i]]++;    }    int count = 0;         // Loop to count for triplets    for(int i = 0; i < n; i++){        for(int j = i + 1; j < n; j++){            if(freq[arr[i] + arr[j]] > 0){                count++;            }        }    }    return count;} // Driver Codepublic static void Main(string[] args){    int n = 4;    int []arr = {1, 5, 3, 2};         // Function Call    Console.WriteLine(countTriplets(arr, n));}} // This code is contributed by Yahs_R
<script> // Javascript implementation to count the// triplets such that the sum of the// two numbers is equal to third number  // Function to find the count of the// triplets such that sum of two// numbers is equal to the third numberfunction countTriplets(arr, n){    let freq = new Uint8Array(100);         // Loop to count the frequency    for (let i=0; i < n; i++){        freq[arr[i]]++;    }    let count = 0;         // Loop to count for triplets    for(let i = 0;i < n; i++){        for(let j = i+1; j < n; j++){            if(freq[arr[i] + arr[j]]){                count++;            }        }    }    return count;} // Driver Code    let n = 4;    let arr = [1, 5, 3, 2];         // Function Call    document.write(countTriplets(arr, n));     //This code is contributed by Mayank Tyagi</script>
2
 
Performance Analysis:  
Time Complexity: O(N2).
Auxiliary Space: O(N).
SURENDRA_GANGWAR
Rajput-Ji
Yash_R
nidhi_biet
mayanktyagi1709
pankajsharmagfg
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