767. Reorganize String

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LeetCode

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Problem

Given a string s, check if the letters can be rearranged so that two characters that are adjacent to each other are not the same.

If possible, output any possible result. If not possible, return the empty string.

Example 1:

Input: s = "aab"
Output: "aba"

Example 2:

Input: s = "aaab"
Output: ""

Constraints:

• 1 <= s.length <= 500
• s consists of lowercase English letters.

Code

class Solution {
    public String reorganizeString(String S) {
        Map<Character, Integer> map = new HashMap<>();
        for (char c : S.toCharArray()) {
            int count = map.getOrDefault(c, 0) + 1;

            if (count > (S.length() + 1) / 2) return "";
            map.put(c, count);
        }

        PriorityQueue<int[]> pq = new PriorityQueue<>((a, b) -> b[1] - a[1]);
        for (char c : map.keySet()) {
            pq.add(new int[]{c, map.get(c)});
        }

        StringBuilder sb = new StringBuilder();
        while (!pq.isEmpty()) {
            int[] first = pq.poll();
            if (sb.length() == 0 || first[0] != sb.charAt(sb.length() - 1)) {
                sb.append((char) first[0]);
                if (--first[1] > 0) {
                    pq.add(first);
                }
            } else {
                int[] second = pq.poll();
                sb.append((char) second[0]);
                if (--second[1] > 0) {
                    pq.add(second);
                }
                pq.add(first);
            }
        }

        return sb.toString();
    }
}