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();
  }
}