Add Leetcode: Restore IP Addresses

Author: aykrieger

README.md

@@ -181,12 +181,14 @@ HackerRank assigns their problems a difficulty rating (Easy, Medium, Hard, Exper
 
 - Employee Names (Easy) [Problem](https://www.hackerrank.com/challenges/name-of-employees/problem) | [Solution](https://github.com/aykrieger/coding-exercises/blob/master/sql/employee-names.sql)
 
-## LeetCode (2 Problems)
+## LeetCode (3 Problems)
 
 - K-diff Pairs in an Array (Medium) [Problem](https://leetcode.com/problems/k-diff-pairs-in-an-array/) | [Solution](https://github.com/aykrieger/coding-exercises/blob/master/src/main/java/leetcode/KDiffPairsInAnArray.java)
 
 - Longest Substring Without Repeating Characters (Medium) [Problem](https://leetcode.com/problems/longest-substring-without-repeating-characters/) | [Solution](https://github.com/aykrieger/coding-exercises/blob/master/src/main/java/leetcode/LongestSubstringWithoutRepeatingCharacters.java)
 
+- Restore IP Addresses (Medium) [Problem](https://leetcode.com/problems/restore-ip-addresses/) | [Solution](https://github.com/aykrieger/coding-exercises/blob/master/src/main/java/leetcode/RestoreIPAddresses.java)
+
 ## aykrieger (1 Problem)
 
 -  First Non-Duplicate [Solution](https://github.com/aykrieger/coding-exercises/blob/master/src/main/java/aykrieger/FirstNonDuplicate.java)

src/main/java/leetcode/RestoreIPAddresses.java

@@ -0,0 +1,51 @@
+package leetcode;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * @see <a href="https://leetcode.com/problems/restore-ip-addresses/">Restore IP Addresses</a>
+ */
+public class RestoreIPAddresses {
+
+  public static List<String> restoreIpAddresses(String input) {
+
+    if (input.length() < 4 || input.length() > 12) {
+      return new ArrayList<>();
+    }
+    List<String> results = new ArrayList<>();
+    List<String> octets = new ArrayList<>();
+    validIPsHelper(results, octets, input, 0, 4);
+    return results;
+  }
+
+  private static void validIPsHelper(List<String> results, List<String> octets, String input,
+      int index,
+      int octetsToAdd) {
+    if (octetsToAdd < 1) {
+      if (index == input.length()) {
+        results.add(String.join(".", octets));
+      }
+      return;
+    }
+
+    int curr = index;
+    int maxIndex = curr + 3;
+    while (curr < input.length() && curr < maxIndex) {
+      String subString = input.substring(index, curr + 1);
+      int num = Integer.parseInt(subString);
+      if (subString.equals(Integer.toString(num)) && isValidOctet(num)) {
+        octets.add(subString);
+        validIPsHelper(results, octets, input, curr + 1, octetsToAdd - 1);
+        octets.remove(octets.size() - 1);
+      } else {
+        break;
+      }
+      curr++;
+    }
+  }
+
+  private static boolean isValidOctet(int num) {
+    return num >= 0 && num <= 255;
+  }
+}

src/test/java/leetcode/RestoreIPAddressesTest.java

@@ -0,0 +1,48 @@
+package leetcode;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+public class RestoreIPAddressesTest {
+
+  @Test
+  public void testRestoreIpAddresses1() {
+    List<String> expected = Arrays.asList("255.255.11.135", "255.255.111.35");
+    List<String> result = RestoreIPAddresses.restoreIpAddresses("25525511135");
+    Collections.sort(expected);
+    Collections.sort(result);
+    assertEquals(expected, result);
+  }
+
+  @Test
+  public void testRestoreIpAddresses2() {
+    List<String> expected = Arrays.asList("0.0.0.0");
+    List<String> result = RestoreIPAddresses.restoreIpAddresses("0000");
+    assertEquals(expected, result);
+  }
+
+  @Test
+  public void testRestoreIpAddresses3() {
+    List<String> expected = Arrays.asList("1.0.10.23", "1.0.102.3", "10.1.0.23", "10.10.2.3",
+        "101.0.2.3");
+    List<String> result = RestoreIPAddresses.restoreIpAddresses("101023");
+    Collections.sort(expected);
+    Collections.sort(result);
+    assertEquals(expected, result);
+  }
+
+  @Test
+  public void testRestoreIpAddresses4() {
+    List<String> expected = Arrays.asList("1.92.168.11", "19.2.168.11", "19.21.68.11",
+        "19.216.8.11", "19.216.81.1", "192.1.68.11", "192.16.8.11", "192.16.81.1", "192.168.1.1");
+
+    List<String> result = RestoreIPAddresses.restoreIpAddresses("19216811");
+    Collections.sort(expected);
+    Collections.sort(result);
+    assertEquals(expected, result);
+  }
+}