Rearrange a string so that all same characters become d distance away

import java.util.*;

public class GfG{
    // Function to check if all identical characters are at least d positions apart
    static boolean valid(String s, int d) {
        for (int i = 0; i < s.length(); i++) {
            for (int j = i + 1; j < s.length(); j++) {
                // Check if characters are the same and too close to each other
                if (s.charAt(i) == s.charAt(j) && Math.abs(i - j) < d) {
                    return false;
                }
            }
        }
        return true;
    }

    static boolean rearrange(String s, int d) {
        // Convert string to character array and sort
        char[] arr = s.toCharArray();
        Arrays.sort(arr);
        s = new String(arr);

        // Try all permutations of the string
        do {
            // For each permutation, check if it satisfies the condition
            if (valid(s, d)) {
                // If valid, print the arrangement and return true
                System.out.println(s);
                return true;
            }
        } while ((s = nextPermutation(s)) != null);

        // If no valid arrangement is found, print a message and return false
        System.out.println("Cannot be rearranged");
        return false;
    }

    static String nextPermutation(String s) {
        char[] arr = s.toCharArray();
        int i = arr.length - 1;
        while (i > 0 && arr[i - 1] >= arr[i]) i--;
        if (i <= 0) return null;
        int j = arr.length - 1;
        while (arr[j] <= arr[i - 1]) j--;
        char temp = arr[i - 1];
        arr[i - 1] = arr[j];
        arr[j] = temp;
        j = arr.length - 1;
        while (i < j) {
            temp = arr[i];
            arr[i] = arr[j];
            arr[j] = temp;
            i++;
            j--;
        }
        return new String(arr);
    }

    public static void main(String[] args) {
        String s = "aabbcc";
        int d = 3;
        rearrange(s, d);
    }
}

import java.util.*;

public class Rearrange {
    public static void rearrange(String s, int d) {
        int n = s.length();

        // Frequency map for characters
        Map<Character, Integer> freq = new HashMap<>();
        for (char ch : s.toCharArray()) {
            freq.put(ch, freq.getOrDefault(ch, 0) + 1);
        }

        // Max heap (sorting by frequency in descending order)
        PriorityQueue<Map.Entry<Character, Integer>> pq = 
            new PriorityQueue<>((a, b) -> b.getValue() - a.getValue());

        pq.addAll(freq.entrySet());

        // Result array
        char[] result = new char[n];
        Arrays.fill(result, '\u0000');

        // Queue to store delayed elements
        Queue<Map.Entry<Character, Integer>> waitQueue = new LinkedList<>();

        int index = 0;

        while (!pq.isEmpty()) {
            Map.Entry<Character, Integer> current = pq.poll();
            char ch = current.getKey();
            int count = current.getValue();

            for (int i = 0; i < count; i++) {
                // Find the next available position
                while (index < n && result[index] != '\u0000') {
                    index++;
                }

                if (index >= n) {
                    System.out.println("Cannot be rearranged");
                    return;
                }

                result[index] = ch;
                index += d;  // Place next occurrence `d` positions apart

                // If index exceeds n, reset it to the
                // first available position
                if (index >= n) {
                    index = 0;
                    while (index < n && result[index] != '\u0000') {
                        index++;
                    }
                }
            }

            if (count > 1) {
                waitQueue.add(new AbstractMap.SimpleEntry<>(ch, count - 1));
            }

            // After `d` iterations, reinsert from the waitQueue
            if (waitQueue.size() >= d) {
                pq.offer(waitQueue.poll());
            }
        }

        System.out.println(new String(result));
    }

    public static void main(String[] args) {
        String s = "aabbcc";
        rearrange(s, 3);
    }
}