计算不超过N的整数，该整数等于任何大于1的整数的至少2次幂

给定正整数N ，任务是计算范围为[1，N]的整数数，该整数可以表示为a ^b ，其中a和b是大于1的整数。

例子：

Input: N = 6
Output: 1
Explanation:
Only such integer from the range [1, 6] is 4 (= 2²).
Therefore, the required count is 1.

Input: N = 10
Output: 3

方法：给定问题可以通过计算所有可能的元素对(a，b)来解决，使得a ^b最多为N。请按照以下步骤解决问题：

• 初始化一个HashSet来存储^b的所有可能值，该值最多为N。
• 遍历范围[2,1√N]，并为的每一个值，至多N，插入一个具有^B值的所有可能值，其中b位于在范围[1，N]。
• 完成上述步骤后，将HashSet的大小打印为整数的结果计数。

下面是上述方法的实现：

Java

// Java program for the above approach
 
import java.util.HashSet;
 
public class GFG {
 
    // Function to count the integers
    // up to N that can be represented
    // as a ^ b, where a &b > 1
    static void printNumberOfPairs(int N)
    {
        // Initialize a HashSet
        HashSet st
            = new HashSet();
 
        // Iterating over the range
        // [2, sqrt(N)]
        for (int i = 2; i * i <= N; i++) {
 
            int x = i;
 
            // Generate all possible
            // power of x
            while (x <= N) {
 
                // Multiply x by i
                x *= i;
 
                // If the generated number
                // lies in the range [1, N]
                // then insert it in HashSet
                if (x <= N) {
                    st.add(x);
                }
            }
        }
 
        // Print the total count
        System.out.println(st.size());
    }
 
    // Driver Code
    public static void main(String args[])
    {
        int N = 10000;
        printNumberOfPairs(N);
    }
}

C#

// C# program for the above approach
using System;
using System.Collections.Generic;
 
class GFG{
 
// Function to count the integers
// up to N that can be represented
// as a ^ b, where a &b > 1
static void printNumberOfPairs(int N)
{
     
    // Initialize a HashSet
    HashSet st = new HashSet();
 
    // Iterating over the range
    // [2, sqrt(N)]
    for(int i = 2; i * i <= N; i++)
    {
        int x = i;
 
        // Generate all possible
        // power of x
        while (x <= N)
        {
             
            // Multiply x by i
            x *= i;
 
            // If the generated number
            // lies in the range [1, N]
            // then insert it in HashSet
            if (x <= N)
            {
                st.Add(x);
            }
        }
    }
 
    // Print the total count
    Console.WriteLine(st.Count);
}
 
// Driver Code
public static void Main(string[] args)
{
    int N = 10000;
    printNumberOfPairs(N);
}
}
 
// This code is contributed by ukasp

输出：

124

时间复杂度： O(N log N)
辅助空间： O(N)