置换系数

排列是指将给定集合的所有成员排列形成序列的过程。一组 n 个元素的排列数由 n! ， 在哪里 ”！”代表阶乘。

由P(n, k)表示的置换系数用于表示从n个元素的集合中获得具有k个元素的有序子集的方式的数量。
在数学上它被给出为：

图片来源：维基
例子 ：

P(10, 2) = 90
P(10, 3) = 720
P(10, 0) = 1
P(10, 1) = 10

系数也可以使用以下递归公式递归计算：

P(n, k) = P(n-1, k) + k* P(n-1, k-1)   

如果我们仔细观察，我们可以分析出这个问题有重叠的子结构，因此我们可以在这里应用动态规划。下面是一个实现相同想法的程序。

C

// A Dynamic Programming based
// solution that uses table P[][]
// to calculate the Permutation
// Coefficient
#include
 
// Returns value of Permutation
// Coefficient P(n, k)
int permutationCoeff(int n, int k)
{
    int P[n + 1][k + 1];
 
    // Calculate value of Permutation
    // Coefficient in bottom up manner
    for (int i = 0; i <= n; i++)
    {
        for (int j = 0; j <= std::min(i, k); j++)
        {
            // Base Cases
            if (j == 0)
                P[i][j] = 1;
 
            // Calculate value using
            // previosly stored values
            else
                P[i][j] = P[i - 1][j] +
                        (j * P[i - 1][j - 1]);
 
            // This step is important
            // as P(i,j)=0 for j>i
            P[i][j + 1] = 0;
        }
    }
    return P[n][k];
}
 
// Driver Code
int main()
{
    int n = 10, k = 2;
    printf("Value of P(%d, %d) is %d ",
            n, k, permutationCoeff(n, k));
    return 0;
}

Java

// Java code for Dynamic Programming based
// solution that uses table P[][] to
// calculate the Permutation Coefficient
import java.io.*;
import java.math.*;
 
class GFG
{
     
    // Returns value of Permutation
    // Coefficient P(n, k)
    static int permutationCoeff(int n,
                                int k)
    {
        int P[][] = new int[n + 2][k + 2];
     
        // Calculate value of Permutation
        // Coefficient in bottom up manner
        for (int i = 0; i <= n; i++)
        {
            for (int j = 0;
                j <= Math.min(i, k);
                j++)
            {
                // Base Cases
                if (j == 0)
                    P[i][j] = 1;
     
                // Calculate value using previosly
                // stored values
                else
                    P[i][j] = P[i - 1][j] +
                            (j * P[i - 1][j - 1]);
     
                // This step is important
                // as P(i,j)=0 for j>i
                P[i][j + 1] = 0;
            }
        }
        return P[n][k];
    }
     
    // Driver Code
    public static void main(String args[])
    {
        int n = 10, k = 2;
        System.out.println("Value of P( " + n + ","+ k +")" +
                        " is " + permutationCoeff(n, k) );
    }
}
 
// This code is contributed by Nikita Tiwari.

Python3

# A Dynamic Programming based
# solution that uses
# table P[][] to calculate the
# Permutation Coefficient
 
# Returns value of Permutation
# Coefficient P(n, k)
def permutationCoeff(n, k):
 
    P = [[0 for i in range(k + 1)]
            for j in range(n + 1)]
 
    # Calculate value of Permutation
    # Coefficient in
    # bottom up manner
    for i in range(n + 1):
        for j in range(min(i, k) + 1):
 
            # Base cases
            if (j == 0):
                P[i][j] = 1
 
            # Calculate value using
            # previously stored values
            else:
                P[i][j] = P[i - 1][j] + (
                        j * P[i - 1][j - 1])
 
            # This step is important
            # as P(i, j) = 0 for j>i
            if (j < k):
                P[i][j + 1] = 0
    return P[n][k]
 
# Driver Code
n = 10
k = 2
print("Value fo P(", n, ", ", k, ") is ",
    permutationCoeff(n, k), sep = "")
 
# This code is contributed by Soumen Ghosh.

C#

// C# code for Dynamic Programming based
// solution that uses table P[][] to
// calculate the Permutation Coefficient
using System;
 
class GFG
{
     
    // Returns value of Permutation
    // Coefficient P(n, k)
    static int permutationCoeff(int n,
                                int k)
    {
        int [,]P = new int[n + 2,k + 2];
     
        // Calculate value of Permutation
        // Coefficient in bottom up manner
        for (int i = 0; i <= n; i++)
        {
            for (int j = 0;
                j <= Math.Min(i, k);
                j++)
            {
                // Base Cases
                if (j == 0)
                    P[i,j] = 1;
     
                // Calculate value using previosly
                // stored values
                else
                    P[i,j] = P[i - 1,j] +
                            (j * P[i - 1,j - 1]);
     
                // This step is important
                // as P(i,j)=0 for j>i
                P[i,j + 1] = 0;
            }
        }
        return P[n,k];
    }
     
    // Driver Code
    public static void Main()
    {
        int n = 10, k = 2;
        Console.WriteLine("Value of P( " + n +
                        ","+ k +")" + " is " +
                        permutationCoeff(n, k) );
    }
}
 
// This code is contributed by anuj_67..

PHP

i
            $P[$i][$j + 1] = 0;
        }
    }
    return $P[$n][$k];
}
 
    // Driver Code
    $n = 10; $k = 2;
    echo "Value of P(",$n," ,",$k,") is ",
            permutationCoeff($n, $k);
 
// This code is contributed by anuj_67.
?>

Javascript

    // Javascript code for Dynamic Programming based
    // solution that uses table P[][] to
    // calculate the Permutation Coefficient
     
    // Returns value of Permutation
    // Coefficient P(n, k)
    function permutationCoeff(n, k)
    {
        let P = new Array(n + 2);
         
        for(let i = 0; i < n + 2; i++)
        {
            P[i] = new Array(k + 2);
        }
      
        // Calculate value of Permutation
        // Coefficient in bottom up manner
        for (let i = 0; i <= n; i++)
        {
            for (let j = 0; j <= Math.min(i, k); j++)
            {
                // Base Cases
                if (j == 0)
                    P[i][j] = 1;
      
                // Calculate value using previosly
                // stored values
                else
                    P[i][j] = P[i - 1][j] + (j * P[i - 1][j - 1]);
      
                // This step is important
                // as P(i,j)=0 for j>i
                P[i][j + 1] = 0;
            }
        }
        return P[n][k];
    }
     
    let n = 10, k = 2;
    document.write("Value of P(" + n + ","+ k +")" + " is " + permutationCoeff(n, k) );
     
    // This code is contributed by decode2207.

输出 ：

Value of P(10, 2) is 90 

在这里我们可以看到时间复杂度为 O(n*k)，空间复杂度为 O(n*k)，因为程序使用辅助矩阵来存储结果。

我们可以在 O(n) 时间内完成吗?
让我们假设我们维护一个单一的一维数组来计算直到 n 的阶乘。我们可以使用计算出的阶乘值并应用公式 P(n, k) = n！ / (nk)!。下面是一个说明相同概念的程序。

C++

// A O(n) solution that uses
// table fact[] to calculate
// the Permutation Coefficient
#include
using namespace std;
 
// Returns value of Permutation
// Coefficient P(n, k)
int permutationCoeff(int n, int k)
{
    int fact[n + 1];
 
    // Base case
    fact[0] = 1;
 
    // Calculate value
    // factorials up to n
    for(int i = 1; i <= n; i++)
    fact[i] = i * fact[i - 1];
 
    // P(n,k) = n! / (n - k)!
    return fact[n] / fact[n - k];
}
 
// Driver Code
int main()
{
    int n = 10, k = 2;
     
    cout << "Value of P(" << n << ", "
        << k << ") is "
        << permutationCoeff(n, k);
 
    return 0;
}
 
// This code is contributed by shubhamsingh10

C

// A O(n) solution that uses
// table fact[] to calculate
// the Permutation Coefficient
#include
 
// Returns value of Permutation
// Coefficient P(n, k)
int permutationCoeff(int n, int k)
{
    int fact[n + 1];
 
    // base case
    fact[0] = 1;
 
    // Calculate value
    // factorials up to n
    for (int i = 1; i <= n; i++)
        fact[i] = i * fact[i - 1];
 
    // P(n,k) = n! / (n - k)!
    return fact[n] / fact[n - k];
}
 
// Driver Code
int main()
{
    int n = 10, k = 2;
    printf ("Value of P(%d, %d) is %d ",
            n, k, permutationCoeff(n, k) );
    return 0;
}

Java

// A O(n) solution that uses
// table fact[] to calculate
// the Permutation Coefficient
import java .io.*;
 
public class GFG {
     
    // Returns value of Permutation
    // Coefficient P(n, k)
    static int permutationCoeff(int n,
                                int k)
    {
        int []fact = new int[n+1];
     
        // base case
        fact[0] = 1;
     
        // Calculate value
        // factorials up to n
        for (int i = 1; i <= n; i++)
            fact[i] = i * fact[i - 1];
     
        // P(n,k) = n! / (n - k)!
        return fact[n] / fact[n - k];
    }
     
    // Driver Code
    static public void main (String[] args)
    {
        int n = 10, k = 2;
        System.out.println("Value of"
        + " P( " + n + ", " + k + ") is "
        + permutationCoeff(n, k) );
    }
}
 
// This code is contributed by anuj_67.

蟒蛇3

# A O(n) solution that uses
# table fact[] to calculate
# the Permutation Coefficient
 
# Returns value of Permutation
# Coefficient P(n, k)
def permutationCoeff(n, k):
    fact = [0 for i in range(n + 1)]
 
    # base case
    fact[0] = 1
 
    # Calculate value
    # factorials up to n
    for i in range(1, n + 1):
        fact[i] = i * fact[i - 1]
 
    # P(n, k) = n!/(n-k)!
    return int(fact[n] / fact[n - k])
 
# Driver Code
n = 10
k = 2
print("Value of P(", n, ", ", k, ") is ",
        permutationCoeff(n, k), sep = "")
 
# This code is contributed
# by Soumen Ghosh

C＃

// A O(n) solution that uses
// table fact[] to calculate
// the Permutation Coefficient
using System;
 
public class GFG {
     
    // Returns value of Permutation
    // Coefficient P(n, k)
    static int permutationCoeff(int n,
                                int k)
    {
        int []fact = new int[n+1];
     
        // base case
        fact[0] = 1;
     
        // Calculate value
        // factorials up to n
        for (int i = 1; i <= n; i++)
            fact[i] = i * fact[i - 1];
     
        // P(n,k) = n! / (n - k)!
        return fact[n] / fact[n - k];
    }
     
    // Driver Code
    static public void Main ()
    {
        int n = 10, k = 2;
        Console.WriteLine("Value of"
        + " P( " + n + ", " + k + ") is "
        + permutationCoeff(n, k) );
    }
}
 
// This code is contributed by anuj_67.

Javascript

    // A O(n) solution that uses
    // table fact[] to calculate
    // the Permutation Coefficient
     
    // Returns value of Permutation
    // Coefficient P(n, k)
    function permutationCoeff(n, k)
    {
        let fact = new Array(n+1);
      
        // base case
        fact[0] = 1;
      
        // Calculate value
        // factorials up to n
        for (let i = 1; i <= n; i++)
            fact[i] = i * fact[i - 1];
      
        // P(n,k) = n! / (n - k)!
        return parseInt(fact[n] / fact[n - k], 10);
    }
     
    let n = 10, k = 2;
    document.write("Value of"
                   + " P(" + n + ", " + k + ") is "
                   + permutationCoeff(n, k) );
                    

输出 ：

Value of P(10, 2) is 90 

AO(n) 时间和 O(1) 额外空间解决方案

C++

// A O(n) time and O(1) extra
// space solution to calculate
// the Permutation Coefficient
#include 
using namespace std;
 
int PermutationCoeff(int n, int k)
{
    int P = 1;
 
    // Compute n*(n-1)*(n-2)....(n-k+1)
    for (int i = 0; i < k; i++)
        P *= (n-i) ;
 
    return P;
}
 
// Driver Code
int main()
{
    int n = 10, k = 2;
    cout << "Value of P(" << n << ", " << k
         << ") is " << PermutationCoeff(n, k);
 
    return 0;
}

Java

// A O(n) time and O(1) extra
// space solution to calculate
// the Permutation Coefficient
import java.io.*;
 
class GFG
{
    static int PermutationCoeff(int n,
                                int k)
    {
        int Fn = 1, Fk = 1;
     
        // Compute n! and (n-k)!
        for (int i = 1; i <= n; i++)
        {
            Fn *= i;
            if (i == n - k)
            Fk = Fn;
        }
        int coeff = Fn / Fk;
        return coeff;
    }
     
    // Driver Code
    public static void main(String args[])
    {
        int n = 10, k = 2;
        System.out.println("Value of P( " + n + "," +
                                         k +") is " +
                            PermutationCoeff(n, k) );
    }
}
     
// This code is contributed by Nikita Tiwari.

C＃

// A O(n) time and O(1) extra
// space solution to calculate
// the Permutation Coefficient
using System;
 
class GFG {
     
    static int PermutationCoeff(int n,
                                int k)
    {
        int Fn = 1, Fk = 1;
     
        // Compute n! and (n-k)!
        for (int i = 1; i <= n; i++)
        {
            Fn *= i;
            if (i == n - k)
                Fk = Fn;
        }
        int coeff = Fn / Fk;
         
        return coeff;
    }
     
    // Driver Code
    public static void Main()
    {
        int n = 10, k = 2;
        Console.WriteLine("Value of P( "
                   + n + "," + k +") is "
              + PermutationCoeff(n, k) );
    }
}
     
// This code is contributed by anuj_67.

Javascript

 
// A O(n) time and O(1) extra
// space solution to calculate
// the Permutation Coefficient   
function PermutationCoeff(n, k)
{
    let P = 1;
 
    // Compute n*(n-1)*(n-2)....(n-k+1)
    for(let i = 0; i < k; i++)
        P *= (n - i);
 
    return P;
}
 
// Driver code
let n = 10, k = 2;
document.write("Value of P(" + n +
                        ", " + k + ") is " +
                        PermutationCoeff(n, k));
                         
// This code is contributed by divyesh072019
 

蟒蛇3

# A O(n) solution that uses
# table fact[] to calculate
# the Permutation Coefficient
 
# Returns value of Permutation
# Coefficient P(n, k)
def permutationCoeff(n, k):
   
  f=1
     
  for i in range(k): #P(n,k)=n*(n-1)*(n-2)*....(n-k-1)
    f*=(n-i)
         
  return f  #This code is contributed by Suyash Saxena
 
# Driver Code
n = 10
k = 2
print("Value of P(", n, ", ", k, ") is ",
        permutationCoeff(n, k))

输出 ：

Value of P(10, 2) is 90 

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