LeetCode 753: Cracking the Safe Solution
Master LeetCode problem 753 (Cracking the Safe), a hard challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.
753. Cracking the Safe
Problem Explanation
Explanation:
To solve this problem, we can use a De Bruijn sequence. A De Bruijn sequence is a cyclic sequence in which every possible k-length sequence occurs exactly once as a contiguous substring.
- Construct a De Bruijn sequence of length k^n + n - 1 over the alphabet [0, k - 1].
- The De Bruijn sequence guarantees that we will find a substring of length n that will unlock the safe.
- Return any substring of length kn - 1 from the De Bruijn sequence.
Time complexity: O(k^n)
Space complexity: O(k^n)
:
Solution Code
class Solution {
public String crackSafe(int n, int k) {
int totalLength = (int) Math.pow(k, n) + n - 1;
StringBuilder sb = new StringBuilder();
StringBuilder start = new StringBuilder();
for (int i = 0; i < n - 1; i++) {
start.append("0");
}
Set<String> visited = new HashSet<>();
dfs(start.toString(), k, n, totalLength, visited, sb);
sb.append(start.toString()); // Append the last n-1 characters to make it cyclic
return sb.toString();
}
private void dfs(String node, int k, int n, int totalLength, Set<String> visited, StringBuilder sb) {
for (int i = 0; i < k; i++) {
String next = node + i;
if (!visited.contains(next)) {
visited.add(next);
dfs(next.substring(1), k, n, totalLength, visited, sb);
sb.append(i);
}
}
}
}Try It Yourself
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