Letter Case Permutation: Accepted

Author: gouthampradhan

README.md

@@ -57,6 +57,7 @@ My accepted leetcode solutions to some of the common interview problems.
 - [Regular Expression Matching](problems/src/backtracking/RegularExpressionMatching.java) (Hard)
 - [Expression Add Operators](problems/src/backtracking/ExpressionAddOperators.java) (Hard)
 - [Wildcard Matching](problems/src/backtracking/WildcardMatching.java) (Hard)
+- [Letter Case Permutation](problems/src/backtracking/LetterCasePermutation.java) (Easy)
 
 #### [Binary Search](problems/src/binary_search)
 

problems/src/backtracking/LetterCasePermutation.java

@@ -0,0 +1,61 @@
+package backtracking;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Created by gouthamvidyapradhan on 12/04/2018.
+ * Given a string S, we can transform every letter individually to be lowercase or uppercase to create another string.
+ * Return a list of all possible strings we could create.
+ * <p>
+ * Examples:
+ * Input: S = "a1b2"
+ * Output: ["a1b2", "a1B2", "A1b2", "A1B2"]
+ * <p>
+ * Input: S = "3z4"
+ * Output: ["3z4", "3Z4"]
+ * <p>
+ * Input: S = "12345"
+ * Output: ["12345"]
+ * Note:
+ * <p>
+ * S will be a string with length at most 12.
+ * S will consist only of letters or digits.
+ *
+ * Solution: O(N x 2 ^ N) Backtrack and generate all possible combinations.
+ */
+public class LetterCasePermutation {
+
+    /**
+     * Main method
+     *
+     * @param args
+     * @throws Exception
+     */
+    public static void main(String[] args) throws Exception {
+        System.out.println(new LetterCasePermutation().letterCasePermutation("a1b2"));
+    }
+
+    public List<String> letterCasePermutation(String S) {
+        List<String> result = new ArrayList<>();
+        backtrack(S, result, 0, "");
+        return result;
+    }
+
+    private void backtrack(String s, List<String> result, int i, String r) {
+        if (i == s.length()) {
+            result.add(r);
+        } else {
+            if (Character.isAlphabetic(s.charAt(i))) {
+                backtrack(s, result, i + 1, r + s.charAt(i));
+                if (Character.isLowerCase(s.charAt(i))) {
+                    backtrack(s, result, i + 1, r + Character.toUpperCase(s.charAt(i)));
+                } else {
+                    backtrack(s, result, i + 1, r + Character.toLowerCase(s.charAt(i)));
+                }
+            } else {
+                backtrack(s, result, i + 1, r + s.charAt(i));
+            }
+        }
+    }
+}