refactor 151

Author: fishercoder1534

README.md

@@ -452,7 +452,7 @@ Your ideas/fixes/algorithms are more than welcome!
 |154|[Find Minimum in Rotated Sorted Array II](https://leetcode.com/problems/find-minimum-in-rotated-sorted-array-ii/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_154.java)| O(logn)|O(1) | Hard| Array, Binary Search
 |153|[Find Minimum in Rotated Sorted Array](https://leetcode.com/problems/find-minimum-in-rotated-sorted-array/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_153.java)| O(logn)|O(1) | Medium| Array, Binary Search
 |152|[Maximum Product Subarray](https://leetcode.com/problems/maximum-product-subarray/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_152.java)| O(n)|O(1) | Medium| Array
-|151|[Reverse Words in a String](https://leetcode.com/problems/reverse-words-in-a-string/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_151.java)| O(n)|O(n) | Medium|
+|151|[Reverse Words in a String](https://leetcode.com/problems/reverse-words-in-a-string/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_151.java)| O(n)|O(n) | Medium| String
 |150|[Evaluate Reverse Polish Notation](https://leetcode.com/problems/evaluate-reverse-polish-notation/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_150.java)| O(?)|O(?) | Medium
 |149|[Max Points on a Line](https://leetcode.com/problems/max-points-on-a-line/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_149.java)| O(?)|O(?) | Hard|
 |148|[Sort List](https://leetcode.com/problems/sort-list/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_148.java) O(nlogn)|O(h) | Medium| Linked List, Sort

src/main/java/com/fishercoder/solutions/_151.java

@@ -1,23 +1,53 @@
 package com.fishercoder.solutions;
-/**Given an input string, reverse the string word by word.
 
+import com.fishercoder.common.utils.CommonUtils;
+
+import java.util.ArrayDeque;
+import java.util.Deque;
+
+/**
+ * 151. Reverse Words in a String
+ * Given an input string, reverse the string word by word.
 For example,
 Given s = "the sky is blue",
-return "blue is sky the".*/
+return "blue is sky the".
+
+ Clarification:
+ What constitutes a word?
+ A sequence of non-space characters constitutes a word.
+ Could the input string contain leading or trailing spaces?
+ Yes. However, your reversed string should not contain leading or trailing spaces.
+ How about multiple spaces between two words?
+ Reduce them to a single space in the reversed string.
+
+ */
 
 public class _151 {
+
     public String reverseWords(String s) {
-        if(!s.contains(" ")) return s;//for cases like this: "a"
-        if(s.matches(" *")) return "";//for cases like this: " "
+        s.trim();
+        if (s == null || s.length() == 0) return "";
         String[] words = s.split(" ");
-        StringBuilder stringBuilder = new StringBuilder();
-        for(int i = words.length-1; i >= 0; i--){
-            if(!words[i].equals("") && !words[i].equals(" ")){
-                stringBuilder.append(words[i]);
-                stringBuilder.append(" ");
+        if (words == null || words.length == 0) return "";
+        Deque<String> stack = new ArrayDeque<>();
+        for (String word : words) {
+            if (!word.equals("")) {
+                stack.offer(word);
             }
         }
-        stringBuilder.deleteCharAt(stringBuilder.length()-1);
-        return stringBuilder.toString();
+        StringBuilder stringBuilder = new StringBuilder();
+        while (!stack.isEmpty()) {
+            stringBuilder.append(stack.pollLast()).append(" ");
+        }
+        return stringBuilder.substring(0, stringBuilder.length()-1).toString();
+    }
+
+    public static void main(String... args) {
+        /**This main program is to demo:
+         * a string that contains consecutive empty spaces when splitting by delimiter " ",
+         * it'll produce a an "" as an element.*/
+        String s = "a b  c";
+        String[] strs = s.split(" ");
+        CommonUtils.printArray_generic_type(strs);
     }
 }

src/test/java/com/fishercoder/_151Test.java

@@ -0,0 +1,36 @@
+package com.fishercoder;
+
+import com.fishercoder.solutions._151;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class _151Test {
+    private static _151 test;
+    private static String s;
+
+    @BeforeClass
+    public static void setup() {
+        test = new _151();
+    }
+
+    @Test
+    public void test1() {
+        s = " ";
+        assertEquals("", test.reverseWords(s));
+    }
+
+    @Test
+    public void test2() {
+        s = " 1";
+        assertEquals("1", test.reverseWords(s));
+    }
+
+    @Test
+    public void test3() {
+        s = "   a   b ";
+        assertEquals("b a", test.reverseWords(s));
+    }
+
+}