Problem:

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"

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:

1. n will be in the range [1, 4].
2. k will be in the range [1, 10].
3. k^n will be at most 4096.

Solution:

　　這道題thumb down的次數大於thumb up的次數，可見這道題不是很好。對於這道題，總共有k^n

Code:

 1 class Solution {
 2 public:
 3     string crackSafe(int n, int k) {
 4         string res = string(n, '0');
 5         unordered_set<string> visited{{res}};
 6         for (int i = 0; i < pow(k, n); ++i) {
 7             string pre = res.substr(res.size() - n + 1, n - 1);
 8             for (int j = k - 1; j >= 0; --j) {
 9                 string cur = pre + to_string(j);
10                 if (!visited.count(cur)) {
11                     visited.insert(cur);
12                     res += to_string(j);
13                     break;
14                 }
15             }
16         }
17         return res;
18     }
19 };