Count triplets (a, b, c) such that a + b, b + c and a + c are all divisible by K | Set 2
Last Updated :
06 Jul, 2021
Given two positive integers N and K, the task is to count the number of triplets (a, b, c) such that 0 < a, b, c < N and (a + b), (b + c) and (c + a) are all multiples of K.
Examples:
Input: N = 2, K = 2
Output: 2
Explanation: All possible triplets that satisfy the given property are (1, 1, 1) and (2, 2, 2).
Therefore, the total count is 2.
Input: N = 3, K = 2
Output: 9
Naive Approach: Refer to the previous post for the simplest approach to solve this problem.
Time Complexity: O(N3)
Auxiliary Space: O(1)
Efficient Approach: The above approach can also be optimized based on the following observations:
- The given condition is expressed by a congruence formula:
=> a + b ? b + c ? c + a ? 0(mod K)
=> a+b ? b+c (mod K)
=> a ? c(mod K)
- The above relation can also be observed without using the congruence formula as:
- As (a + b) is a multiple of K and (c + b) is a multiple of K. Therefore, (a + b) ? (c + b) = a – c is also a multiple of K, i.e., a ? b ? c (mod K).
- Therefore, the expression can be further evaluated to:
=> a + b ? 0 (mod K)
=> a + a ? 0 (mod K) (since a is congruent to b)
=> 2a ? 0 (mod K)
From the above observations, the result can be calculated for the following two cases:
- If K is odd, then a ? b ? c ? 0(mod K) since all three are congruent and the total number of triplets can be calculated as (N / K)3.
- If K is even, then K is divisible by 2 and a ? 0 (mod K), b ? 0 (mod K), and c ? 0 (mod K). Therefore, the total number of triplets can be calculated as (N / K)3 ((N + (K/2)) / K)3.
Below is the implementation of the above approach:
C++
#include "bits/stdc++.h"
using namespace std;
int countTriplets( int N, int K)
{
if (K % 2 == 0) {
long long int x = N / K;
long long int y = (N + (K / 2)) / K;
return x * x * x + y * y * y;
}
else {
long long int x = N / K;
return x * x * x;
}
}
int main()
{
int N = 2, K = 2;
cout << countTriplets(N, K);
return 0;
}
|
Java
import java.io.*;
import java.lang.*;
import java.util.*;
class GFG{
static int countTriplets( int N, int K)
{
if (K % 2 == 0 )
{
int x = N / K;
int y = (N + (K / 2 )) / K;
return x * x * x + y * y * y;
}
else
{
int x = N / K;
return x * x * x;
}
}
public static void main(String[] args)
{
int N = 2 , K = 2 ;
System.out.print(countTriplets(N, K));
}
}
|
Python3
def countTriplets(N, K):
if (K % 2 = = 0 ):
x = N / / K
y = (N + (K / / 2 )) / / K
return x * x * x + y * y * y
else :
x = N / / K
return x * x * x
if __name__ = = "__main__" :
N = 2
K = 2
print (countTriplets(N, K))
|
Javascript
<script>
function countTriplets(N, K)
{
if (K % 2 == 0)
{
var x = parseInt(N / K);
var y = parseInt((N + (K / 2)) / K);
return x * x * x + y * y * y;
}
else
{
var x = parseInt(N / K);
return x * x * x;
}
}
var N = 2, K = 2;
document.write(countTriplets(N, K));
</script>
|
C#
using System;
class GFG{
static int countTriplets( int N, int K)
{
if (K % 2 == 0)
{
int x = N / K;
int y = (N + (K / 2)) / K;
return x * x * x + y * y * y;
}
else
{
int x = N / K;
return x * x * x;
}
}
static void Main() {
int N = 2, K = 2;
Console.Write(countTriplets(N, K));
}
}
|
Javascript
<script>
function countTriplets(N,K)
{
if (K % 2 == 0) {
let x = parseInt(N / K, 10);
let y = parseInt((N + parseInt(K / 2, 10)) / K, 10);
return x * x * x + y * y * y;
}
else {
let x = parseInt(N / K, 10);
return x * x * x;
}
}
let N = 2, K = 2;
document.write(countTriplets(N, K));
</script>
|
Time Complexity: O(1)
Auxiliary Space: O(1)
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