RearrangeKDistanceApart.java

/**
 * 
 * Example 1:

Input: s = "aabbcc", k = 3
Output: "abcabc"
Explanation: The same letters are at least a distance of 3 from each other.
Example 2:

Input: s = "aaabc", k = 3
Output: ""
Explanation: It is not possible to rearrange the string.
Example 3:

Input: s = "aaadbbcc", k = 2
Output: "abacabcd"
Explanation: The same letters are at least a distance of 2 from each other.
 

**/

class Solution {
    public String rearrangeString(String s, int k) {
        int n = s.length();
        int[] cnt = new int[26];
        for (char c : s.toCharArray()) {
            ++cnt[c - 'a'];
        }
        PriorityQueue<int[]> pq = new PriorityQueue<>((a, b) -> b[0] - a[0]);
        for (int i = 0; i < 26; ++i) {
            if (cnt[i] > 0) {
                pq.offer(new int[] {cnt[i], i});
            }
        }
        Deque<int[]> q = new ArrayDeque<>();
        StringBuilder ans = new StringBuilder();
        while (!pq.isEmpty()) {
            var p = pq.poll();
            int v = p[0], c = p[1];
            ans.append((char) ('a' + c));
            q.offer(new int[] {v - 1, c});
            if (q.size() >= k) {
                p = q.pollFirst();
                if (p[0] > 0) {
                    pq.offer(p);
                }
            }
        }
        return ans.length() == n ? ans.toString() : "";
    }
}