Count of triples (A, B, C) where A*C is greater than B*B

Given three integers A, B and C. The task is to count the number of triples (a, b, c) such that a * c > b2, where 0 < a <= A, 0 < b <= B and 0 < c <= C.

Examples:

Input: A = 3, B = 2, C = 2
Output: 6
Following triples are counted :
(1, 1, 2), (2, 1, 1), (2, 1, 2), (3, 1, 1), (3, 1, 2) and (3, 2, 2).



Input: A = 3, B = 3, C = 3
Output: 11

Naive approach:
The brute force approach is to consider all possible triples (a, b, c) and count those triples that satisfy the constraint a*c > b2.

Below is the implementation of the given approach.

C++

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// C++ implementation
#include <bits/stdc++.h>
using namespace std;
  
// function to return the count
// of the valid triplets
long long countTriplets(int A, int B, int C)
{
    long long ans = 0;
    for (int i = 1; i <= A; i++) {
        for (int j = 1; j <= B; j++) {
            for (int k = 1; k <= C; k++) {
                if (i * k > j * j)
                    ans++;
            }
        }
    }
    return ans;
}
  
// Driver Code
int main()
{
    int A, B, C;
    A = 3, B = 2, C = 2;
  
    // function calling
    cout << countTriplets(A, B, C);
}

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Java

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// Java implementation of above approach
import java.util.*;
  
class GFG
{
  
// function to return the count
// of the valid triplets
static long countTriplets(int A, int B, int C)
{
    long ans = 0;
    for (int i = 1; i <= A; i++) 
    {
        for (int j = 1; j <= B; j++)
        {
            for (int k = 1; k <= C; k++) 
            {
                if (i * k > j * j)
                    ans++;
            }
        }
    }
    return ans;
}
  
// Driver Code
public static void main (String[] args)
{
    int A = 3, B = 2, C = 2;
  
    // function calling
    System.out.println(countTriplets(A, B, C));
}
}
  
// This code is contributed by PrinciRaj1992

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Python3

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# Python3 implementation for above approach
  
# function to return the count
# of the valid triplets
def countTriplets(A, B, C):
    ans = 0
    for i in range(1, A + 1):
        for j in range(1, B + 1):
            for k in range(1, C + 1):
                if (i * k > j * j):
                    ans += 1
  
    return ans
  
# Driver Code
A = 3
B = 2
C = 2
  
# function calling
print(countTriplets(A, B, C))
  
# This code is contributed by Mohit Kumar

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C#

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// C# implementation of above approach
using System;
  
class GFG
{
  
// function to return the count
// of the valid triplets
static long countTriplets(int A, 
                          int B, int C)
{
    long ans = 0;
    for (int i = 1; i <= A; i++) 
    {
        for (int j = 1; j <= B; j++)
        {
            for (int k = 1; k <= C; k++) 
            {
                if (i * k > j * j)
                    ans++;
            }
        }
    }
    return ans;
}
  
// Driver Code
public static void Main (String[] args)
{
    int A = 3, B = 2, C = 2;
  
    // function calling
    Console.WriteLine(countTriplets(A, B, C));
}
}
      
// This code is contributed by 29AjayKumar

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Output:

6

Time Complexity: O(A*B*C).

Efficient approach:

Let us count all triplets for a given value of b = k for all k from 1 to B.

  1. For a given b = k we need to find all a = i and c = j that satisfy i * j > k2
  2. For a = i, find smallest c = j that satisfies the condition.

    Since c = j satisfies this condition therefore c = j + 1, c = j + 2, … and so on, will also satisfy the condition.
    So we can easily count all triples in which a = i and b = k.

  3. Also if for some a = i, c = j is the smallest value such that the given condition is satisfied so it can be observed that a = j and all c >= i also satisfy the condition.

    The condition is also satisfied by a = j + 1 and c >= i that is all values a >= j and c >= i also satisfy the condition.

  4. The above observation helps us to count all triples in which b = k and a >= j easily.
  5. Now we need to count all triples in which b = k and i < a < j.
  6. Thus for a given value of b = k we only need to go upto a = square root of k.

Below is the implementation of the above approach:

C++

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// C++ implementation
#include <bits/stdc++.h>
using namespace std;
  
// Counts the number of triplets
// for a given value of b
long long getCount(int A, int B2,
                   int C)
{
    long long count = 0;
  
    // Count all triples in which a = i
    for (int i = 1; i <= A; i++) {
  
        // Smallest value j
        // such that i*j > B2
        long long j = (B2 / i) + 1;
  
        // Count all (i, B2, x)
        // such that x >= j
        if (C >= j)
            count = (count + C - j + 1);
  
        // count all (x, B2, y) such
        // that x >= j this counts
        // all such triples in
        // which a >= j
        if (A >= j && C >= i)
            count = (count
                     + (C - i + 1)
                           * (A - j + 1));
  
        // As all triples with a >= j
        // have been counted reduce
        // A to j - 1.
        if (A >= j)
            A = j - 1;
    }
    return count;
}
  
// Counts the number of triples that
// satisfy the given constraints
long long countTriplets(int A, int B,
                        int C)
{
    long long ans = 0;
    for (int i = 1; i <= B; i++) {
  
        // GetCount of triples in which b = i
        ans = (ans
               + getCount(A, i * i, C));
    }
    return ans;
}
  
// Driver Code
int main()
{
    int A, B, C;
    A = 3, B = 2, C = 2;
  
    // Function calling
    cout << countTriplets(A, B, C);
}

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Java

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// Java implementation of the approach
import java.util.*;
  
class GFG 
{
  
// Counts the number of triplets
// for a given value of b
static long getCount(int A, int B2, int C)
{
    long count = 0;
  
    // Count all triples in which a = i
    for (int i = 1; i <= A; i++)
    {
  
        // Smallest value j
        // such that i*j > B2
        long j = (B2 / i) + 1;
  
        // Count all (i, B2, x)
        // such that x >= j
        if (C >= j)
            count = (count + C - j + 1);
  
        // count all (x, B2, y) such
        // that x >= j this counts
        // all such triples in
        // which a >= j
        if (A >= j && C >= i)
            count = (count + (C - i + 1) *
                             (A - j + 1));
  
        // As all triples with a >= j
        // have been counted reduce
        // A to j - 1.
        if (A >= j)
            A = (int) (j - 1);
    }
    return count;
}
  
// Counts the number of triples that
// satisfy the given constraints
static long countTriplets(int A, int B, int C)
{
    long ans = 0;
    for (int i = 1; i <= B; i++)
    {
  
        // GetCount of triples in which b = i
        ans = (ans + getCount(A, i * i, C));
    }
    return ans;
}
  
// Driver Code
public static void main(String[] args)
{
    int A, B, C;
    A = 3; B = 2; C = 2;
  
    // Function calling
    System.out.println(countTriplets(A, B, C));
}
}
  
// This code is contributed by Princi Singh

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C#

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// C# implementation of the approach
using System;
using System.Collections.Generic;                 
      
class GFG 
{
  
// Counts the number of triplets
// for a given value of b
static long getCount(int A, int B2, int C)
{
    long count = 0;
  
    // Count all triples in which a = i
    for (int i = 1; i <= A; i++)
    {
  
        // Smallest value j
        // such that i*j > B2
        long j = (B2 / i) + 1;
  
        // Count all (i, B2, x)
        // such that x >= j
        if (C >= j)
            count = (count + C - j + 1);
  
        // count all (x, B2, y) such
        // that x >= j this counts
        // all such triples in
        // which a >= j
        if (A >= j && C >= i)
            count = (count + (C - i + 1) *
                             (A - j + 1));
  
        // As all triples with a >= j
        // have been counted reduce
        // A to j - 1.
        if (A >= j)
            A = (int) (j - 1);
    }
    return count;
}
  
// Counts the number of triples that
// satisfy the given constraints
static long countTriplets(int A, int B, int C)
{
    long ans = 0;
    for (int i = 1; i <= B; i++)
    {
  
        // GetCount of triples in which b = i
        ans = (ans + getCount(A, i * i, C));
    }
    return ans;
}
  
// Driver Code
public static void Main(String[] args)
{
    int A, B, C;
    A = 3; B = 2; C = 2;
  
    // Function calling
    Console.WriteLine(countTriplets(A, B, C));
}
}
  
// This code is contributed by Princi Singh

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Output:

6

Time Complexity: O(B^{2})



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Final year BTech IT student at DTU, Upcoming Technology Analyst at Morgan Stanley

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