Cracking The Safe

Author: gouthampradhan

README.md

@@ -110,7 +110,8 @@ My accepted leetcode solutions to some of the common interview problems.
 - [Number of Distinct Islands II](problems/src/depth_first_search/NumberOfDistinctIslandsII.java) (Hard)
 - [Smallest Rectangle Enclosing Black Pixels](problems/src/depth_first_search/SmallestRectangleEnclosingBlackPixels.java) (Hard)
 - [Bricks Falling When Hit](problems/src/depth_first_search/BricksFallingWhenHit.java) (Hard)
-- [Bricks Falling When Hit](problems/src/depth_first_search/RobotRoomCleaner.java) (Hard)
+- [Robot Room Cleaner](problems/src/depth_first_search/RobotRoomCleaner.java) (Hard)
+- [Cracking the Safe](problems/src/depth_first_search/CrackingTheSafe.java) (Hard)
 
 #### [Design](problems/src/design)
 

problems/src/depth_first_search/CrackingTheSafe.java

@@ -0,0 +1,99 @@
+package depth_first_search;
+
+import java.util.HashSet;
+import java.util.Set;
+
+/**
+ * Created by gouthamvidyapradhan on 09/03/2019
+ * There is a box protected by a password. The password is n digits, where each letter can be one of the first k
+ * digits 0, 1, ..., k-1.
+ *
+ * You can keep inputting the password, the password will automatically be matched against the last n digits entered.
+ *
+ * For example, assuming the password is "345", I can open it when I type "012345", but I enter a total of 6 digits.
+ *
+ * Please return any string of minimum length that is guaranteed to open the box after the entire string is inputted.
+ *
+ * Example 1:
+ * Input: n = 1, k = 2
+ * Output: "01"
+ * Note: "10" will be accepted too.
+ * Example 2:
+ * Input: n = 2, k = 2
+ * Output: "00110"
+ * Note: "01100", "10011", "11001" will be accepted too.
+ * Note:
+ * n will be in the range [1, 4].
+ * k will be in the range [1, 10].
+ * k^n will be at most 4096.
+ *
+ * Solution O(n x k ^ n) Do a dfs and explore every possible states which form a n digit number with-in the given
+ * range k. Maintain a 'result' string and keep appending the new digit in every state, if the total number of states
+ * visited reaches k ^ n then, the result string will be the answer.
+ */
+public class CrackingTheSafe {
+
+    /**
+     * Main method
+     *
+     * @param args
+     */
+    public static void main(String[] args) {
+        System.out.println(new CrackingTheSafe().crackSafe(4, 5));
+    }
+
+    public String crackSafe(int n, int k) {
+        int states = getStates(n, k);
+        int[] N = new int[k];
+        for (int i = 0; i < k; i++) {
+            N[i] = i;
+        }
+        return generate(N, n, 0, 0, "", k, states);
+    }
+
+    private int getStates(int n, int k) {
+        if (n == 0) return 1;
+        if (n == 1) return k;
+        int result = 1;
+        for (int i = 0; i < n; i++) {
+            result *= k;
+        }
+        return result;
+    }
+
+    private String generate(int[] N, int n, int i, int count, String num, int k, int states) {
+        if (count == n) {
+            return dfs(num, new StringBuilder(num), new HashSet<>(), k, states, 1);
+        } else {
+            for (int j = i; j < N.length; j++) {
+                String result = generate(N, n, j, count + 1, num + String.valueOf(N[j]), k, states);
+                if (!result.isEmpty()) {
+                    return result;
+                }
+            }
+        }
+        return "";
+    }
+
+    private String dfs(String num, StringBuilder result, Set<String> done, int k, int states, int count) {
+        done.add(num);
+        if (states == count) {
+            return result.toString();
+        } else {
+            for (int i = 0; i < k; i++) {
+                String newNum = (num + String.valueOf(i));
+                String newState = newNum.substring(1);
+                if (!done.contains(newState)) {
+                    String retValue = dfs(newState, result.append(String.valueOf(i)), done, k, states, count + 1);
+                    if (!retValue.isEmpty()) {
+                        return retValue;
+                    } else {
+                        result.deleteCharAt(result.length() - 1);
+                    }
+                }
+            }
+        }
+        done.remove(num);
+        return "";
+    }
+}