451. Sort Characters By Frequency

Author: zhaoguanchen

README.md

@@ -257,6 +257,7 @@ The project is divided into two parts: `structure` and `solution`.
 
 | No.  | Title                                                        | Difficulty | Solution                                                     | Idea                                     |
 | ---- | ------------------------------------------------------------ | ---------- | ------------------------------------------------------------ | ---------------------------------------- |
+| 451  | [Sort Characters By Frequency](https://leetcode.com/problems/sort-characters-by-frequency/) | Medium     | [SortCharactersByFrequency.java](src/leetcode/solution/Hash/SortCharactersByFrequency.java) | Map and Sort. Or Bucket Sort.            |
 | 567  | [Longest Consecutive Sequence](https://leetcode.com/problems/longest-consecutive-sequence/) | Medium     | [LongestConsecutiveSequence.java](src/leetcode/solution/Hash/LongestConsecutiveSequence.java) | Hash Table: using HashSet save the nums. |
 
 #### [Graph](src/leetcode/solution/Graph)

src/leetcode/solution/Hash/SortCharactersByFrequency.java

@@ -0,0 +1,102 @@
+package leetcode.solution.Hash;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * 451. Sort Characters By Frequency
+ */
+public class SortCharactersByFrequency {
+
+
+    public static void main(String[] args) {
+        String s = "wqaqaaasddd";
+        SortSolution sortSolution = new SortSolution();
+        String ans = sortSolution.frequencySort(s);
+        System.out.println(ans);
+        // aaaadddqqsw
+
+        BucketSolution bucketSolution = new BucketSolution();
+        ans = bucketSolution.frequencySort(s);
+        System.out.println(ans);
+    }
+
+}
+
+class SortSolution {
+
+
+    public String frequencySort(String s) {
+        Map<Character, Integer> count = new HashMap<>();
+        char[] chars = s.toCharArray();
+
+        for (char c : chars) {
+            count.put(c, count.getOrDefault(c, 0) + 1);
+        }
+
+        List<Character> list = new ArrayList<>(count.keySet());
+
+        list.sort((a, b) -> {
+            return count.get(b) - count.get(a);
+        });
+
+        StringBuilder sb = new StringBuilder();
+
+        for (char c : list) {
+            int num = count.get(c);
+            for (int i = 0; i < num; i++) {
+                sb.append(c);
+            }
+        }
+
+        return sb.toString();
+
+    }
+}
+
+
+class BucketSolution {
+
+
+    public String frequencySort(String s) {
+        Map<Character, Integer> count = new HashMap<>();
+        char[] chars = s.toCharArray();
+
+        for (char c : chars) {
+            count.put(c, count.getOrDefault(c, 0) + 1);
+        }
+
+        int max = 0;
+        for (Integer v : count.values()) {
+            max = Math.max(max, v);
+        }
+
+        List<List<Character>> buckets = new ArrayList<>();
+
+        for (int i = 0; i < max + 1; i++) {
+            buckets.add(new ArrayList<>());
+        }
+
+        for (Map.Entry<Character, Integer> entry : count.entrySet()) {
+            buckets.get(entry.getValue()).add(entry.getKey());
+        }
+
+        StringBuilder sb = new StringBuilder();
+
+        for (int i = buckets.size() - 1; i >= 0; i--) {
+            List<Character> list = buckets.get(i);
+            if (!list.isEmpty()) {
+                for (Character c : list) {
+                    for (int j = 0; j < i; j++) {
+                        sb.append(c);
+                    }
+                }
+            }
+        }
+
+        return sb.toString();
+
+    }
+}