[LEET-451] add 451

fishercoder1534 · fishercoder1534 · commit 2a6f1521b1d4 · 2017-02-15T09:02:29.000-08:00
diff --git a/leetcode-algorithms/README.md b/leetcode-algorithms/README.md
@@ -24,6 +24,7 @@
 |455|[Assign Cookies](https://leetcode.com/problems/assign-cookies/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/AssignCookies.java)| O(n)|O(1) | Easy|
 |454|[4Sum II](https://leetcode.com/problems/4sum-ii/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/_4SumII.java) | O(n) |O(n) | Medium| HashMap
 |453|[Minimum Moves to Equal Array Elements](https://leetcode.com/problems/minimum-moves-to-equal-array-elements/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/MinimumMovestoEqualArrayElements.java)| O(n)|O(1) | Easy|
+|451|[Sort Characters By Frequency](https://leetcode.com/problems/sort-characters-by-frequency/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/SortCharactersByFrequency.java) | O(nlogn) |O(n) | Medium| HashMap
 |448|[Find All Numbers Disappeared in an Array](https://leetcode.com/problems/find-all-numbers-disappeared-in-an-array/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/FindAllNumbersDisappearedinanArray.java)| O(n)|O(1) | Easy| Array, HashMap
 |447|[Number of Boomerangs](https://leetcode.com/problems/number-of-boomerangs/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/NumberofBoomerangs.java)| O(n^2)|O(n) | Easy| HashMap
 |442|[Find All Duplicates in an Array](https://leetcode.com/problems/find-all-duplicates-in-an-array/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/FindAllDuplicatesinanArray.java)| O(n)|O(1) | Medium| Array
diff --git a/leetcode-algorithms/src/main/java/com/stevesun/solutions/SortCharactersByFrequency.java b/leetcode-algorithms/src/main/java/com/stevesun/solutions/SortCharactersByFrequency.java
@@ -0,0 +1,60 @@
+package com.stevesun.solutions;
+
+import java.util.*;
+
+/**
+ * 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.
+
+ */
+public class SortCharactersByFrequency {
+
+    public String frequencySort(String s) {
+        Map<Character, Integer> map = new HashMap();
+        for (char c : s.toCharArray()) map.put(c, map.getOrDefault(c, 0) + 1);
+        List<Map.Entry<Character, Integer>> list = new ArrayList<>(map.entrySet());
+        Collections.sort(list, (o1, o2) -> (o2.getValue()).compareTo(o1.getValue()));
+        StringBuilder stringBuilder = new StringBuilder();
+        for (Map.Entry<Character, Integer> entry : list) {
+            for (int i = 0; i < entry.getValue(); i++) {
+                stringBuilder.append(entry.getKey());
+            }
+        }
+        return stringBuilder.toString();
+    }
+}
diff --git a/leetcode-algorithms/src/test/java/com/stevesun/SortCharactersByFrequencyTest.java b/leetcode-algorithms/src/test/java/com/stevesun/SortCharactersByFrequencyTest.java
@@ -0,0 +1,53 @@
+package com.stevesun;
+
+import com.stevesun.solutions.SortCharactersByFrequency;
+import org.junit.Before;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import static junit.framework.Assert.assertEquals;
+
+/**
+ * Created by stevesun on 1/15/17.
+ */
+public class SortCharactersByFrequencyTest {
+    private static SortCharactersByFrequency test;
+    private static String expected;
+    private static String actual;
+    private static String input;
+
+    @BeforeClass
+    public static void setup(){
+        test = new SortCharactersByFrequency();
+    }
+
+    @Before
+    public void setupForEachTest(){
+        expected = "";
+        actual = "";
+    }
+
+    @Test
+    public void test1(){
+        input = "tree";
+        expected = "eert";
+        actual = test.frequencySort(input);
+        assertEquals(expected, actual);
+    }
+
+    @Test
+    public void test2(){
+        input = "cccaaa";
+        expected = "aaaccc";
+        actual = test.frequencySort(input);
+        assertEquals(expected, actual);
+    }
+
+    @Test
+    public void test3(){
+        input = "Aabb";
+        expected = "bbAa";
+        actual = test.frequencySort(input);
+        assertEquals(expected, actual);
+    }
+}
