frequcy sort chars

Author: jackzhenguo

HashTable/HashTable.Lib/FrequSortSln.cs

@@ -0,0 +1,83 @@
+/* ==============================================================================
+ * 功能描述：FrequSort  
+ * 创 建 者：gz
+ * 创建日期：2017/5/10 18:05:01
+ * ==============================================================================*/
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+//Given a string, sort it in decreasing order based on the frequency of characters.
+
+//Example 1:
+
+//Input:
+//"tree"
+
+//Output:
+//"eert"
+
+//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:
+
+//Input:
+//"cccaaa"
+
+//Output:
+//"cccaaa"
+
+//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.
+//Example 3:
+
+//Input:
+//"Aabb"
+
+//Output:
+//"bbAa"
+
+//Explanation:
+//"bbaA" is also a valid answer, but "Aabb" is incorrect.
+//Note that 'A' and 'a' are treated as two different characters.
+namespace FrequencySortSln
+{
+    /// <summary>
+    /// FrequSort
+    /// </summary>
+    public class FrequSortSln
+    {
+        public string FrequencySort(string s)
+        {
+            Dictionary<char, int> freq = new Dictionary<char, int>();
+            string[] buckets = new string[s.Length+1]; //索引用作字符个数，最大的个数索引也为最大
+            //count char frequency
+            foreach (var c in s)
+            {
+                if (!freq.ContainsKey(c))
+                    freq.Add(c, 1);
+                else freq[c]++;
+            }
+            //put char to buckets
+            foreach (var item in freq)
+            {
+                int n = item.Value;
+                char c = item.Key;
+                StringBuilder builder = new StringBuilder();
+                buckets[n] += builder.Append(c, n).ToString();
+            }
+            StringBuilder builder2 = new StringBuilder();
+            //form string
+            for (int i = s.Length; i > 0; i--)
+            {
+                if (buckets[i] != null)
+                    builder2.Append(buckets[i]);
+            }
+            return builder2.ToString();
+        }
+
+    }
+}