Added a challenge to make a string URL Friendly

Author: Geektimus

README.md

@@ -65,6 +65,12 @@ We need to write a function that will tell us if all the characters on a given s
 ### Check permutation
 We need to write a function that given two strings let us find if one is a permutation of the other.
 
+### URL Friendly String
+Given a String and its true length, transform the string to a URL friendly string by replacing the spaces by %20
+
+**Note:** We can assume that the string contains a length that allow us to store the additional chars
+without having to allocate a new string.
+
 ### Others (Challenges proposed by Friends or Solved on hackatons)
 #### Recursively reverse a string
 Given a string, reverse the characters and return

src/main/java/com/codingmaniacs/others/StringChallenges.java

@@ -39,4 +39,44 @@ static String findFirstRepeatedChar(String str) {
         }
         return "";
     }
+
+    /**
+     * Given a String and its true length, transform the string to a URL friendly string by replacing the spaces by %20
+     *
+     * @param str        String to be transformed
+     * @param trueLength True size of the string without the trailing spaces.
+     * @return A URL friendly string with the spaces replaced.
+     */
+    static String URLlify(String str, int trueLength) {
+        if (str.length() == trueLength)
+            return str;
+
+        char[] chars = str.toCharArray();
+        int spaces = 0;
+
+        // Count spaces
+        for (int i = 0; i < trueLength; i++) {
+            if (chars[i] == ' ') {
+                spaces++;
+            }
+        }
+
+        if (trueLength < str.length()) chars[trueLength] = '\0';
+
+        // Replace the string backwards so we don't need to worry about overriding data.
+        int index = trueLength + spaces * 2;
+        for (int j = trueLength - 1; j >= 0; j--) {
+            if (chars[j] == ' ') {
+                chars[index - 1] = '0';
+                chars[index - 2] = '2';
+                chars[index - 3] = '%';
+                index = index - 3;
+            } else {
+                chars[index - 1] = chars[j];
+                index--;
+            }
+        }
+
+        return new String(chars);
+    }
 }

src/test/java/com/codingmaniacs/others/StringChallengesTests.java

@@ -95,4 +95,34 @@ public void findFirstOccurrenceOfCharLongerString() {
         String result = StringChallenges.findFirstRepeatedChar(str);
         assertEquals("Since the letter a is repeated once, that's the solution", expected, result);
     }
+
+    @Test
+    public void transformStringToURLNoSpaces() {
+        String str = "helloworld";
+        int trueLength = 10;
+        String expected = "helloworld";
+
+        String result = StringChallenges.URLlify(str, trueLength);
+        assertEquals("We don't have spaces to parse so we expect the same string", expected, result);
+    }
+
+    @Test
+    public void transformStringToURLTwoSpaces() {
+        String str = "hello world! example    ";
+        int trueLength = 20;
+        String expected = "hello%20world!%20example";
+
+        String result = StringChallenges.URLlify(str, trueLength);
+        assertEquals(expected, result);
+    }
+
+    @Test
+    public void transformStringToURLFourSpaces() {
+        String str = "this is my friendly command        ";
+        int trueLength = 27;
+        String expected = "this%20is%20my%20friendly%20command";
+
+        String result = StringChallenges.URLlify(str, trueLength);
+        assertEquals(expected, result);
+    }
 }