https://www.geeksforgeeks.org/rearrange-characters-string-no-two-adjacent/

RearrangeCharacters.java

class Solution {
	public static String rearrangeCharacters(String str) {
	    Map<Character, Integer> freq = new HashMap<>();
        for (char c : str.toCharArray()) freq.put(c, freq.getOrDefault(c, 0) + 1);
        List<Map.Entry<Character, Integer>> freqList = new ArrayList<>(freq.entrySet());
//         freqList.sort(Map.Entry.comparingByValue());
        freqList.sort((e1, e2) -> e2.getValue() - e1.getValue());
        Map<Character, Integer> sortedFreq = new LinkedHashMap<>();
        for (Map.Entry<Character, Integer> entry : freqList) sortedFreq.put(entry.getKey(), entry.getValue());
        int n = str.length();
        char[] result = new char[n];
        int index = 0;
        for (Map.Entry<Character, Integer> entry : sortedFreq.entrySet()) {
            char key = entry.getKey();
            int value = entry.getValue();
            while (value -- > 0) {
                result[index % n] = key;
                index += 2;
            }
        }
        return new String(result);
	}
}