从N的值中找到N！

给定一个代表数字N阶乘的数字K ，任务是找到N的值。
注意： K <10 ¹⁸

例子：

Input: K = 120
Output: 5
Explanation:
5! = 1 * 2 * 3 * 4 * 5 = 120

Input: K = 6
Output: 3
Explanation:
3! = 1 * 2 * 3 = 6

方法：给出N的值！小于10 ¹⁸ 。通过观察我们可以看到，仅当N <= 18时，这才是正确的。因此，我们可以从1到18预先计算阶乘的值，并将其存储在哈希表或映射中。在预计算之后，对于N！的每个值，将在恒定时间内返回对应的N。

下面是上述方法的实现：

C++

// C++ program to find a number such that
// the factorial of that number is given
  
#include "bits/stdc++.h"
#define ll long long int
using namespace std;
  
// Map to precompute and store the
// factorials of the numbers
map m;
  
// Function to precompute factorial
int precompute()
{
  
    ll fact = 1;
    for (ll i = 1; i <= 18; i++) {
  
        // Calculating the factorial for
        // each i and storing in a map
        fact = fact * i;
        m[fact] = i;
    }
}
  
// Driver code
int main()
{
    // Precomputing the factorials
    precompute();
  
    int K = 120;
    cout << m[K] << endl;
  
    K = 6;
    cout << m[K] << endl;
  
    return 0;
}

Java

// Java program to find a number such that
// the factorial of that number is given
import java.util.*;
  
class GFG{
   
// Map to precompute and store the
// factorials of the numbers
static Map m = new HashMap();
   
// Function to precompute factorial
static void precompute()
{
   
    int fact = 1;
    for (int i = 1; i <= 18; i++) {
   
        // Calculating the factorial for
        // each i and storing in a map
        fact = fact * i;
        m.put(fact, i);
    }
}
   
// Driver code
public static void main(String[] args)
{
    // Precomputing the factorials
    precompute();
   
    int K = 120;
    System.out.print(m.get(K) +"\n");
   
    K = 6;
    System.out.print(m.get(K) +"\n");
   
}
}
  
// This code is contributed by 29AjayKumar

Python3

# Python3 program to find a number such that 
# the factorial of that number is given 
  
# Map to precompute and store the 
# factorials of the numbers 
m = {}; 
  
# Function to precompute factorial 
def precompute() :
  
    fact = 1; 
    for i in range(1, 19) :
  
        # Calculating the factorial for 
        # each i and storing in a map 
        fact = fact * i; 
        m[fact] = i; 
          
# Driver code 
if __name__ == "__main__" :
  
    # Precomputing the factorials 
    precompute(); 
  
    K = 120; 
    print(m[K]); 
  
    K = 6; 
    print(m[K]) ; 
  
# This code is contributed by AnkitRai01

C#

// C# program to find a number such that
// the factorial of that number is given
using System;
using System.Collections.Generic;
  
class GFG{
    
// Map to precompute and store the
// factorials of the numbers
static Dictionary m = new Dictionary();
    
// Function to precompute factorial
static void precompute()
{
    
    int fact = 1;
    for (int i = 1; i <= 18; i++) {
    
        // Calculating the factorial for
        // each i and storing in a map
        fact = fact * i;
        m.Add(fact, i);
    }
}
    
// Driver code
public static void Main(String[] args)
{
    // Precomputing the factorials
    precompute();
    
    int K = 120;
    Console.Write(m[K] +"\n");
    
    K = 6;
    Console.Write(m[K] +"\n");  
}
}
  
// This code is contributed by 29AjayKumar

输出：

5
3