LeetCode-in-Java

753. Cracking the Safe

Hard

There is a safe protected by a password. The password is a sequence of n digits where each digit can be in the range [0, k - 1].

The safe has a peculiar way of checking the password. When you enter in a sequence, it checks the most recent n digits that were entered each time you type a digit.

• For example, the correct password is "345" and you enter in "012345":
  • After typing 0, the most recent 3 digits is "0", which is incorrect.
  • After typing 1, the most recent 3 digits is "01", which is incorrect.
  • After typing 2, the most recent 3 digits is "012", which is incorrect.
  • After typing 3, the most recent 3 digits is "123", which is incorrect.
  • After typing 4, the most recent 3 digits is "234", which is incorrect.
  • After typing 5, the most recent 3 digits is "345", which is correct and the safe unlocks.

Return any string of minimum length that will unlock the safe at some point of entering it.

Example 1:

Input: n = 1, k = 2

Output: “10”

Explanation: The password is a single digit, so enter each digit. “01” would also unlock the safe.

Example 2:

Input: n = 2, k = 2

Output: “01100”

Explanation: For each possible password:

• “00” is typed in starting from the 4^th digit.
• “01” is typed in starting from the 1^st digit.
• “10” is typed in starting from the 3^rd digit.
• “11” is typed in starting from the 2^nd digit.

Thus “01100” will unlock the safe. “01100”, “10011”, and “11001” would also unlock the safe.

Constraints:

• 1 <= n <= 4
• 1 <= k <= 10
• 1 <= kn <= 4096

Solution

public class Solution {
    private String foundStr;

    public String crackSafe(int n, int k) {
        int targetCnt = (int) Math.pow(k, n);
        boolean[] visited = new boolean[(int) Math.pow(10, n)];
        visited[0] = true;
        int visitedCnt = 1;
        StringBuilder crackStr = new StringBuilder();
        crackStr.append("0".repeat(Math.max(0, n)));
        dfsAddPwd(n, k, crackStr, 0, visited, visitedCnt, targetCnt);
        return foundStr;
    }

    private void dfsAddPwd(
            int n,
            int k,
            StringBuilder crackStr,
            int prev,
            boolean[] visited,
            int visitedCnt,
            int targetCnt) {
        if (foundStr != null) {
            return;
        }
        if (visitedCnt == targetCnt) {
            foundStr = crackStr.toString();
            return;
        }
        int root = 10 * prev % ((int) Math.pow(10, n));
        for (int i = 0; i < k; i++) {
            int current = root + i;
            if (!visited[current]) {
                crackStr.append(i);
                visited[current] = true;
                dfsAddPwd(n, k, crackStr, current, visited, visitedCnt + 1, targetCnt);
                crackStr.setLength(crackStr.length() - 1);
                visited[current] = false;
            }
        }
    }
}

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