Find maximum N such that the sum of square of first N natural numbers is not more than X

Given an integer X, the task is to find the maximum value N such that the sum of first N natural numbers is not more than X.

Examples:

Input: X = 5
Output: 2
2 is the maximum possible value of N because for N = 3, the sum of the series will exceed X
i.e. 12 + 22 + 32 = 1 + 4 + 9 = 14

Input: X = 25
Output: 3



Simple Solution: A simple solution is to run a loop from 1 till the maximum N such that S(N) ≤ X, where S(N) is the sum of square of first N natural numbers. Sum of square of first N natural numbers is given by the formula S(N) = N * (N + 1) * (2 * N + 1) / 6. The time complexity of this approach is O(N).

Efficient Approach: An efficient solution is to use Binary Search to find the value of N. The time complexity of this approach is O(log N).

Below is the implementation of the above approach:

C++

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// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
#define ll long long
  
// Function to return the sum of the squares
// of first N natural numbers
ll squareSum(ll N)
{
    ll sum = (ll)(N * (N + 1) * (2 * N + 1)) / 6;
  
    return sum;
}
  
// Function to return the maximum N such that
// the sum of the squares of first N
// natural numbers is not more than X
ll findMaxN(ll X)
{
    ll low = 1, high = 100000;
    int N = 0;
  
    while (low <= high) {
        ll mid = (high + low) / 2;
  
        if (squareSum(mid) <= X) {
            N = mid;
            low = mid + 1;
        }
  
        else
            high = mid - 1;
    }
  
    return N;
}
  
// Driver code
int main()
{
    ll X = 25;
    cout << findMaxN(X);
  
    return 0;
}

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Java

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// Java implementation of the approach
class GFG 
{
  
// Function to return the sum of the squares
// of first N natural numbers
static long squareSum(long N)
{
    long sum = (long)(N * (N + 1) * (2 * N + 1)) / 6;
  
    return sum;
}
  
// Function to return the maximum N such that
// the sum of the squares of first N
// natural numbers is not more than X
static long findMaxN(long X)
{
    long low = 1, high = 100000;
    int N = 0;
  
    while (low <= high) 
    {
        long mid = (high + low) / 2;
  
        if (squareSum(mid) <= X) 
        {
            N = (int) mid;
            low = mid + 1;
        }
  
        else
            high = mid - 1;
    }
  
    return N;
}
  
// Driver code
public static void main(String[] args)
{
    long X = 25;
    System.out.println(findMaxN(X));
}
}
  
// This code contributed by Rajput-Ji

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C#

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// C# implementation of the approach
using System;
  
class GFG
{
      
// Function to return the sum of the squares
// of first N natural numbers
static long squareSum(long N)
{
    long sum = (long)(N * (N + 1) * (2 * N + 1)) / 6;
  
    return sum;
}
  
// Function to return the maximum N such that
// the sum of the squares of first N
// natural numbers is not more than X
static long findMaxN(long X)
{
    long low = 1, high = 100000;
    int N = 0;
  
    while (low <= high) 
    {
        long mid = (high + low) / 2;
  
        if (squareSum(mid) <= X) 
        {
            N = (int) mid;
            low = mid + 1;
        }
  
        else
            high = mid - 1;
    }
  
    return N;
}
  
// Driver code
static public void Main ()
{
          
    long X = 25;
    Console.WriteLine(findMaxN(X));
}
}
  
// This code contributed by ajit

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Python3

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# Python3 implementation of the approach 
  
# Function to return the sum of the 
# squares of first N natural numbers 
def squareSum(N): 
  
    Sum = (N * (N + 1) * (2 * N + 1)) // 6
    return Sum
  
# Function to return the maximum N such 
# that the sum of the squares of first N 
# natural numbers is not more than X 
def findMaxN(X):
  
    low, high, N = 1, 100000, 0
  
    while low <= high:
        mid = (high + low) // 2
  
        if squareSum(mid) <= X: 
            N = mid 
            low = mid + 1
          
        else:
            high = mid - 1
      
    return
  
# Driver code 
if __name__ == "__main__"
  
    X = 25
    print(findMaxN(X)) 
  
# This code is contributed 
# by Rituraj Jain

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PHP

Output:

3


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Improved By : rituraj_jain, Rajput-Ji, jit_t