451. Sort Characters By Frequency

Reference: LeetCode
Difficulty: Medium

My Post: Java Solution Summary of HashMap + Sorting / Buckets (why not using TreeSet?)

Problem

Given a string, sort it in decreasing order based on the frequency of characters.

Example:

Input:
"tree"
Output:
"eert"

Input:
"cccaaa"

Output:
"cccaaa"

Analysis

HashMap + Sorting

• Build the map to count frequency.
• Build an array list to sort elements by frequency.
• Build the string.

Note: Actually you can also use PriorityQueue to sort.

public String frequencySort(String s) {
  if (s == null) {
    return null;
  }
  
  Map<Character, Integer> map = new HashMap<>();
  
  for (int i = 0; i < s.length(); ++i) {
    Character c = s.charAt(i);
    map.put(c, map.getOrDefault(c, 0) + 1);
  }
  
  List<Character> arrayList = new ArrayList<>(map.keySet());
  Collections.sort(arrayList, (n1, n2) -> (map.get(n2) - map.get(n1)));
  
  // build the string
  StringBuilder sb = new StringBuilder();
  for (Character c : arrayList) {
    int count = map.get(c);
    while (count > 0) {
      sb.append(c);
      count -= 1;
    }
  }
  
  return sb.toString();
}

Time: $O(N\log{M})$ where $M$ is the number of distinct characters.
Space: $O(N)$

• $O(M)$ the array and the map (with greater overhead).
• $O(N)$ the output string.

Using TreeSet here is not a good idea!

The code will not give correct results. In the tree example, r will be missing. It is because equals and compareTo methods should be consistent. Since we have a comparator (n1, n2) - > (map.get(n2)- map.get(n1)), when t and r have the same count, they are treated as the “same” characters in the set. Thus, t won’t be added into the set.

public String frequencySort(String s) {
  if (s == null) {
    return null;
  }
  
  Map<Character, Integer> map = new HashMap<>();
  Comparator<Character> comp = (n1, n2) -> (map.get(n2) - map.get(n1));
  Set<Character> set = new TreeSet<>(comp);
  
  for (int i = 0; i < s.length(); ++i) {
    Character c = s.charAt(i);
    map.put(c, map.getOrDefault(c, 0) + 1); // before set
    set.add(c);
  }
  
  // build the string
  StringBuilder sb = new StringBuilder();
  for (Character c : set) {
    int count = map.get(c);
    while (count > 0) {
      sb.append(c);
      count -= 1;
    }
  }
  
  return sb.toString();
}

Bucket Sort

We can use n buckets to store characters with specific frequencies. It is a typical way to use much space to gain running time improvement.

Note:

public String frequencySort(String s) {
  int n = s.length();
  Map<Character, Integer> map = new HashMap<>();
  
  // Count frequency
  for (int i = 0; i < s.length(); ++i) {
    Character c = s.charAt(i);
    map.put(c, map.getOrDefault(c, 0) + 1);
  }

  List<Character>[] buckets = new ArrayList[n + 1]; // or 256
  
  // Build buckets
  for (char c: map.keySet()) {
    int count = map.get(c);
    if (buckets[count] == null) {
      buckets[count] = new ArrayList<>();
    }
    buckets[count].add(c);
  }
  
  // Build output string
  StringBuilder sb = new StringBuilder();
  
  for (int i = n; i >= 0; --i) {
    if (buckets[i] != null) {
      continue;
    }
    for (char c : buckets[i]) {
      int count = i;
      while (count > 0) {
        sb.append(c);
        count -= 1;
      }
    }
  }
  return sb.toString();
}

Time: $O(N)$
Space: $O(N)$

• $O(M)$ the buckets and the map (with greater overhead).
• $O(N)$ the output string.