@@ -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)
+- [ing the Safe](problems/src/depth_first_search/ingTheSafe.java) (Hard)
 
 #### [Design](problems/src/design)
 

@@ -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 ingTheSafe {
+
+/**
+* Main method
+*
+* @param args
+*/
+public static void main(String[] args) {
+System.out.println(new ingTheSafe().Safe(4, 5));
+}
+
+public String Safe(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 "";
+}
+}