给定长度的第 K 个字典字符串

给定两个整数N和K ，任务是按字典顺序找到长度为N 的^第K^个字符串。如果长度为N的可能字符串的数量小于K ，则打印-1 。

例子：

Input: N = 3, K = 10
Output: “aaj”
Explanation: The 10th string in the lexicographical order starting from “aaa” is “aaj”.

Input: N = 2, K = 1000
Output: -1
Explanation: A total of 26*26 = 676 strings of length 2 are possible. So the output will be -1.

方法：

让我们假设一个长度为N的字符串是一个以 26 为底的整数。

例如，如果 N = 3，第一个字符串将是s = “aaa”，其整数形式的 base 26 表示将是0 0 0 ，第二个字符串将是s = “aab” ，base 26 表示将是0 0 1和很快。因此，每个数字都可以在[0, 25]内有一个值。

从最后一位开始，我们需要将其更改为所有可能的值，然后是倒数第二位，依此类推。

所以，简化的问题是找到一个十进制数K到一个 26 基数的表示。您可以阅读这篇文章以清楚地了解如何执行此操作。

在找到以 26 为底的整数形式的答案后，我们会将每个数字转换为其等效字符，其中0是‘a’ ， 1是‘b’ ，…。 24是‘y’ ， 25是‘z’ 。

下面是上述方法的实现：

C++

// C++ program to find the K-th 
// lexicographical string of length N
#include 
using namespace std;
  
string find_kth_String_of_n(int n, int k) 
{
      
    // Initialize the array to store
    // the base 26 representation of  
    // K with all zeroes, that is,  
    // the initial String consists
    // of N a's
    int d[n] = {0};
  
    // Start filling all the
    // N slots for the base 26
    // representation of K
    for(int i = n - 1; i > -1; i--) 
    {
          
       // Store the remainder
       d[i] = k % 26;
         
       // Reduce K
       k /= 26;
    }
      
    // If K is greater than the
    // possible number of strings
    // that can be represented by
    // a string of length N
    if (k > 0)
        return "-1";
  
    string s = "";
      
    // Store the Kth lexicographical
    // string
    for(int i = 0; i < n; i++)
       s += (d[i] + ('a'));
  
    return s;
}
  
// Driver Code
int main()
{
    int n = 3;
    int k = 10;
      
    // Reducing k value by 1 because
    // our stored value starts from 0
    k -= 1;
      
    cout << find_kth_String_of_n(n, k);
    return 0;
}
  
// This code is contributed by 29AjayKumar

Java

// Java program to find the
// K-th lexicographical String
// of length N
class GFG{
      
static String find_kth_String_of_n(int n, int k) 
{
    // Initialize the array to
    // store the base 26
    // representation of K
    // with all zeroes, that is,
    // the initial String consists
    // of N a's
    int[] d = new int[n];
  
    // Start filling all the
    // N slots for the base 26
    // representation of K
    for (int i = n - 1; i > -1; i--) 
    {
  
        // Store the remainder
        d[i] = k % 26;
  
        // Reduce K
        k /= 26;
    }
      
    // If K is greater than the
    // possible number of Strings
    // that can be represented by
    // a String of length N
    if (k > 0)
        return "-1";
  
    String s = "";
    // Store the Kth lexicographical
    // String
    for (int i = 0; i < n; i++)
        s += (char)(d[i] + ('a'));
  
    return s;
}
  
public static void main(String[] args) 
{
    int n = 3;
    int k = 10;
      
    // Reducing k value by 1 because
    // our stored value starts from 0
    k -= 1;
    System.out.println(find_kth_String_of_n(n, k));
}
}
  
// This code is contributed by PrinciRaj1992

Python3

# Python program to find the
# K-th lexicographical string
# of length N
def find_kth_string_of_n(n, k):
    # Initialize the array to
    # store the base 26 
    # representation of K
    # with all zeroes, that is,
    # the initial string consists 
    # of N a's
    d =[0 for i in range(n)]
  
    # Start filling all the 
    # N slots for the base 26
    # representation of K
    for i in range(n - 1, -1, -1):
  
        # Store the remainder
        d[i]= k % 26
  
        # Reduce K
        k//= 26
  
    # If K is greater than the
    # possible number of strings
    # that can be represented by 
    # a string of length N
    if k>0:
        return -1
      
      
    s =""
    # Store the Kth lexicographical 
    # string
    for i in range(n):
        s+= chr(d[i]+ord('a'))
      
    return s     
n = 3
k = 10
# Reducing k value by 1 because 
# our stored value starts from 0
k-= 1 
print(find_kth_string_of_n(n, k))

C#

// C# program to find the
// K-th lexicographical String
// of length N
using System;
class GFG{
      
static String find_kth_String_of_n(int n, int k) 
{
    // Initialize the array to
    // store the base 26
    // representation of K
    // with all zeroes, that is,
    // the initial String consists
    // of N a's
    int[] d = new int[n];
  
    // Start filling all the
    // N slots for the base 26
    // representation of K
    for (int i = n - 1; i > -1; i--) 
    {
  
        // Store the remainder
        d[i] = k % 26;
  
        // Reduce K
        k /= 26;
    }
      
    // If K is greater than the
    // possible number of Strings
    // that can be represented by
    // a String of length N
    if (k > 0)
        return "-1";
  
    string s = "";
      
    // Store the Kth lexicographical
    // String
    for (int i = 0; i < n; i++)
        s += (char)(d[i] + ('a'));
  
    return s;
}
  
// Driver Code
public static void Main() 
{
    int n = 3;
    int k = 10;
      
    // Reducing k value by 1 because
    // our stored value starts from 0
    k -= 1;
    Console.Write(find_kth_String_of_n(n, k));
}
}
  
// This code is contributed by Code_Mech

输出：

aaj

时间复杂度： O(N)

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