给定集合中包含元素X的子集的计数

给定一个由N个唯一正整数组成的数组arr []和一个元素X ，任务是计算存在元素X的给定集合的子集的总数。

例子：

Input: arr[] = [4, 5, 6, 7], X = 5
Output: 8
Explanation:
All subsets in which element 5 is present are:
{5}, {4, 5}, {5, 6}, {5, 7}, {4, 5, 6}, {4, 5, 7}, {5, 6, 7}, {4, 5, 6, 7}

Input: arr[] = [1, 2, 3], X = 1
Output: 4
Explanation:
All subsets in which element 1 is present are:
{1}, {1, 2}, {1, 3}, {1, 2, 3}

天真的方法：
一种简单的解决方案是生成给定集合的所有可能子集，它们均为2 ^ n，其中n是给定集合的大小，并对元素X存在的子集的数量进行计数。

下面是上述方法的实现。

C++

// C++ code to implement the above approach
  
#include 
using namespace std;
  
int CountSubSet(int arr[], int n, int X)
{
    // N stores total number of subsets
    int N = pow(2, n);
    int count = 0;
  
    // Generate each subset one by one
    for (int i = 0; i < N; i++) {
  
        // Check every bit of i
        for (int j = 0; j < n; j++) {
  
            // if j'th bit of i is set,
            // check arr[j] with X
            if (i & (1 << j))
                if (arr[j] == X)
                    count += 1;
        }
    }
    return count;
}
  
// Driver code
int main()
{
    int arr[] = { 4, 5, 6, 7 };
    int X = 5;
    int n = sizeof(arr) / sizeof(arr[0]);
  
    cout << CountSubSet(arr, n, X);
  
    return 0;
}

Java

// Java code to implement the above approach
class GFG
{
  
static int CountSubSet(int arr[], int n, int X)
{
    // N stores total number of subsets
    int N = (int) Math.pow(2, n);
    int count = 0;
  
    // Generate each subset one by one
    for (int i = 0; i < N; i++)
    {
  
        // Check every bit of i
        for (int j = 0; j < n; j++) 
        {
  
            // if j'th bit of i is set,
            // check arr[j] with X
            if ((i & (1 << j)) != 0)
                if (arr[j] == X)
                    count += 1;
        }
    }
    return count;
}
  
// Driver code
public static void main(String[] args)
{
    int arr[] = { 4, 5, 6, 7 };
    int X = 5;
    int n = arr.length;
  
    System.out.print(CountSubSet(arr, n, X));
}
}
  
// This code is contributed by PrinciRaj1992

Python3

# Python3 code to implement the above approach 
def CountSubSet(arr, n, X) : 
  
    # N stores total number of subsets 
    N = 2 ** n;
    count = 0; 
  
    # Generate each subset one by one 
    for i in range(N) :
  
        # Check every bit of i 
        for j in range(n) :
  
            # if j'th bit of i is set, 
            # check arr[j] with X 
            if (i & (1 << j)) :
                if (arr[j] == X) :
                    count += 1; 
      
    return count; 
  
# Driver code 
if __name__ == "__main__" :
  
    arr = [ 4, 5, 6, 7 ]; 
    X = 5; 
    n = len(arr); 
  
    print(CountSubSet(arr, n, X)); 
  
# This code is contributed by AnkitRai01

C#

// C# code to implement the above approach
using System;
  
class GFG
{
  
static int CountSubSet(int []arr, int n, int X)
{
    // N stores total number of subsets
    int N = (int) Math.Pow(2, n);
    int count = 0;
  
    // Generate each subset one by one
    for (int i = 0; i < N; i++)
    {
  
        // Check every bit of i
        for (int j = 0; j < n; j++) 
        {
  
            // if j'th bit of i is set,
            // check arr[j] with X
            if ((i & (1 << j)) != 0)
                if (arr[j] == X)
                    count += 1;
        }
    }
    return count;
}
  
// Driver code
public static void Main(String[] args)
{
    int []arr = { 4, 5, 6, 7 };
    int X = 5;
    int n = arr.Length;
  
    Console.Write(CountSubSet(arr, n, X));
}
}
  
// This code is contributed by 29AjayKumar

输出：

8

时间复杂度： O(n * 2 ^ n)。

高效的解决方案：

• 一个有效的解决方案是使用集合中的每个元素都精确地存在于2 ^(n-1)个子集中的事实。
• 在此，在此解决方案中，首先检查给定值X是否存在于给定的一组元素中。
• 如果存在X，则计算并返回2 ^(n-1)(使用模幂计算2 ^ n-1的幂)。
• 否则，返回0。

下面是上述方法的实现

C++

// C++ implementation of above approach
#include 
using namespace std;
  
// Function to calculate (2^(n-1))
int calculatePower(int b, int e)
  
{
  
    // Initially initialize answer to 1
    int ans = 1;
  
    while (e > 0) {
  
        // If e is odd,
        // multiply b with answer
        if (e % 2 == 1)
            ans = ans * b;
  
        e = e / 2;
        b = b * b;
    }
    return ans;
}
  
// Function to count subsets in which
// X element is present
int CountSubSet(int arr[], int n, int X)
{
    int count = 0, checkX = 0;
  
    // Check if X is present in
    // given subset or not
    for (int i = 0; i < n; i++) {
  
        if (arr[i] == X) {
            checkX = 1;
            break;
        }
    }
  
    // If X is present in set
    // then calculate 2^(n-1) as count
    if (checkX == 1)
        count = calculatePower(2, n - 1);
  
    // if X is not present in a given set
    else
        count = 0;
  
    return count;
}
  
// Driver Function
int main()
{
    int arr[] = { 4, 5, 6, 7 };
    int X = 5;
    int n = sizeof(arr) / sizeof(arr[0]);
  
    cout << CountSubSet(arr, n, X);
  
    return 0;
}

Java

// Java implementation of above approach
class GFG 
{
      
    // Function to calculate (2^(n-1)) 
    static int calculatePower(int b, int e) 
    { 
      
        // Initially initialize answer to 1 
        int ans = 1; 
        while (e > 0) 
        { 
      
            // If e is odd, 
            // multiply b with answer 
            if (e % 2 == 1) 
                ans = ans * b; 
      
            e = e / 2; 
            b = b * b; 
        } 
        return ans; 
    } 
      
    // Function to count subsets in which 
    // X element is present 
    static int CountSubSet(int arr[], int n, int X) 
    { 
        int count = 0, checkX = 0; 
      
        // Check if X is present in 
        // given subset or not 
        for (int i = 0; i < n; i++)
        { 
            if (arr[i] == X)
            { 
                checkX = 1; 
                break; 
            } 
        } 
      
        // If X is present in set 
        // then calculate 2^(n-1) as count 
        if (checkX == 1) 
            count = calculatePower(2, n - 1); 
      
        // if X is not present in a given set 
        else
            count = 0; 
      
        return count; 
    } 
      
    // Driver code 
    public static void main (String[] args)
    { 
        int arr[] = { 4, 5, 6, 7 }; 
        int X = 5; 
        int n = arr.length; 
      
        System.out.println(CountSubSet(arr, n, X)); 
    } 
}
  
// This code is contributed by AnkitRai01

Python3

# Python3 implementation of above approach 
  
# Function to calculate (2^(n-1)) 
def calculatePower(b, e) :
  
    # Initially initialize answer to 1 
    ans = 1; 
  
    while (e > 0) :
  
        # If e is odd, 
        # multiply b with answer 
        if (e % 2 == 1) :
            ans = ans * b; 
  
        e = e // 2; 
        b = b * b; 
      
    return ans; 
  
# Function to count subsets in which 
# X element is present 
def CountSubSet(arr, n, X) : 
  
    count = 0; checkX = 0; 
  
    # Check if X is present in 
    # given subset or not 
    for i in range(n) : 
  
        if (arr[i] == X) :
            checkX = 1; 
            break; 
  
    # If X is present in set 
    # then calculate 2^(n-1) as count 
    if (checkX == 1) :
        count = calculatePower(2, n - 1); 
  
    # if X is not present in a given set 
    else :
        count = 0; 
  
    return count; 
  
# Driver code 
if __name__ == "__main__" : 
  
    arr = [ 4, 5, 6, 7 ]; 
    X = 5; 
    n = len(arr); 
  
    print(CountSubSet(arr, n, X)); 
  
# This code is contributed by AnkitRai01

C＃

// C# implementation of above approach
using System;
  
class GFG 
{
      
    // Function to calculate (2^(n-1)) 
    static int calculatePower(int b, int e) 
    { 
      
        // Initially initialize answer to 1 
        int ans = 1; 
        while (e > 0) 
        { 
      
            // If e is odd, 
            // multiply b with answer 
            if (e % 2 == 1) 
                ans = ans * b; 
      
            e = e / 2; 
            b = b * b; 
        } 
        return ans; 
    } 
      
    // Function to count subsets in which 
    // X element is present 
    static int CountSubSet(int []arr, int n, int X) 
    { 
        int count = 0, checkX = 0; 
      
        // Check if X is present in 
        // given subset or not 
        for (int i = 0; i < n; i++)
        { 
            if (arr[i] == X)
            { 
                checkX = 1; 
                break; 
            } 
        } 
      
        // If X is present in set 
        // then calculate 2^(n-1) as count 
        if (checkX == 1) 
            count = calculatePower(2, n - 1); 
      
        // if X is not present in a given set 
        else
            count = 0; 
      
        return count; 
    } 
      
    // Driver code 
    public static void Main()
    { 
        int []arr = { 4, 5, 6, 7 }; 
        int X = 5; 
        int n = arr.Length; 
      
        Console.WriteLine(CountSubSet(arr, n, X)); 
    } 
}
  
// This code is contributed by AnkitRai01

输出：

8

时间复杂度： O(n)

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