SortCharByFrequency.java

package hashing;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

/**
 * Created by gouthamvidyapradhan on 25/03/2017.
 * Given a string, sort it in decreasing order based on the frequency of characters.
 * <p>
 * Example 1:
 * <p>
 * Input:
 * "tree"
 * <p>
 * Output:
 * "eert"
 * <p>
 * Explanation:
 * 'e' appears twice while 'r' and 't' both appear once.
 * So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer.
 * Example 2:
 * <p>
 * Input:
 * "cccaaa"
 * <p>
 * Output:
 * "cccaaa"
 * <p>
 * Explanation:
 * Both 'c' and 'a' appear three times, so "aaaccc" is also a valid answer.
 * Note that "cacaca" is incorrect, as the same characters must be together.
 * <p>
 * Example 3:
 * <p>
 * Input:
 * "Aabb"
 * <p>
 * Output:
 * "bbAa"
 * <p>
 * Explanation:
 * "bbaA" is also a valid answer, but "Aabb" is incorrect.
 * Note that 'A' and 'a' are treated as two different characters.
 */
public class SortCharByFrequency {
    class Freq {
        int i;
        int c;
    }

    private int[] buff = new int[256];

    /**
     * Main method
     *
     * @param args
     * @throws Exception
     */
    public static void main(String[] args) throws Exception {
        System.out.println(new SortCharByFrequency().frequencySort("askdfkasdkfasdkljfklasdjfkl"));
    }

    public String frequencySort(String s) {
        if (s == null || s.isEmpty()) return s;
        Arrays.fill(buff, 0);
        StringBuilder sb = new StringBuilder();
        for (int i = 0, l = s.length(); i < l; i++)
            buff[s.charAt(i)]++;

        List<Freq> fList = new ArrayList<>();
        for (int i = 0; i < 256; i++) {
            if (buff[i] > 0) {
                Freq f = new Freq();
                f.i = i;
                f.c = buff[i];
                fList.add(f);
            }
        }

        Collections.sort(fList, (o1, o2) -> Integer.compare(o2.c, o1.c));

        for (Freq f : fList) {
            char c = (char) f.i;
            int freq = f.c;
            while (freq-- > 0)
                sb.append(c);
        }
        return sb.toString();
    }
}