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Recursion_KthPermutationOfSequence.java
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Recursion_KthPermutationOfSequence.java
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import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
// https://www.youtube.com/watch?v=W9SIlE2jhBQ (good video explanation)
// https://gist.github.com/SuryaPratapK/a64499da7bfd8f4e3d3c2dd744e9a904 (code)
// https://leetcode.com/problems/permutation-sequence/
// https://leetcode.com/problems/permutation-sequence/discuss/22507/%22Explain-like-I'm-five%22-Java-Solution-in-O(n)
// https://www.youtube.com/watch?v=knTd0fgAo-0
// https://www.geeksforgeeks.org/find-the-k-th-permutation-sequence-of-first-n-natural-numbers/
public class Recursion_KthPermutationOfSequence {
private static String kthPermutation(int n , int k) {
// The block size nth digit will be the factorial of n
long[] factorials = getFactorials(n);
List<Character> characters = getListOfCharacters(n);
k = k-1;
String op = "";
for(int i=1; i<=n; i++) {
long blockSize = factorials[n-i];
int index = (int) (k/blockSize);
op += characters.get(index);
characters.remove(index);
k = k - ((int) (index*blockSize));
}
return op;
}
private static long[] getFactorials(int n) {
long[] factorials = new long[n+1];
long fact = 1;
factorials[0] = 1;
for(int i = 1; i <= n ; i++) {
fact *= i;
factorials[i] = fact;
}
return factorials;
}
private static List<Character> getListOfCharacters(int n) {
List<Character> characters = new ArrayList<>();
for(int i = 0; i <n ; i++) {
characters.add((char) (65 + i));
}
return characters;
}
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int n , k;
n = scanner.nextInt();
k = scanner.nextInt();
String ans = kthPermutation(n , k);
for(int i = 0 ; i < ans.length() ; i++) {
System.out.print(ans.charAt(i));
}
}
}