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:
nwill be in the range[1, 4].kwill be in the range[1, 10].k^nwill be at most4096.
Hint:
We can think of this problem as the problem of finding an Euler path (a path visiting every edge exactly once) on the following graph: there are $$k^{n-1}$$ nodes with each node having $$k$$ edges. It turns out this graph always has an Eulerian circuit (path starting where it ends.) We should visit each node in "post-order" so as to not get stuck in the graph prematurely.
连续输入一串密码字符，每次输入一个会按最后一个字符开始匹配。给定密码长度为n，数字个数为k，k的值为0到k-1，用[0, k-1]的数字组成一个密码，使得这个密码一定能打开盒子，返回长度最短的一个密码。
思路：如果保证一定能打开箱子，输入的字符串要能覆盖所有的长度为n的字符串。其实是找到所有由[0, k-1]的数字组成的长度为n的组合，总共有k^n个组合。
解法：DFS
goal: to find the minimum String that matches all combinations of 1..k with length n.
to build the graph:
node: all possible combinations, e.g. 00, 01, 11, 10;
edge: if the last n - 1 digits of node1 can be transformed to node2 by appending a digit from 1..k, e.g. 01 => (1 + 1) => 11, there will be an edge between node1 and node2 ;
transition:
For each node, we pick its last n - 1 digits. if the digits picked can be transfromed to a valid combination, i.e., another node which has not been visited by appending one more digit from 1..k, we go further following the same pattern... we stop when we have visited all the possible combinations, i.e., all nodes.
start node: 0...0 (n 0's) e.g. 00 for n = 2;
end: when visited.size() == k^n;
Java:
class Solution {
public String crackSafe(int n, int k) {
StringBuilder sb = new StringBuilder();
int total = (int) (Math.pow(k, n));
for (int i = 0; i < n; i++) sb.append('0');
Set visited = new HashSet<>();
visited.add(sb.toString());
dfs(sb, total, visited, n, k);
return sb.toString();
}
private boolean dfs(StringBuilder sb, int goal, Set visited, int n, int k) {
if (visited.size() == goal) return true;
String prev = sb.substring(sb.length() - n + 1, sb.length());
for (int i = 0; i < k; i++) {
String next = prev + i;
if (!visited.contains(next)) {
visited.add(next);
sb.append(i);
if (dfs(sb, goal, visited, n, k)) return true;
else {
visited.remove(next);
sb.delete(sb.length() - 1, sb.length());
}
}
}
return false;
}
}
Java:
public String crackSafe(int n, int k) {
StringBuilder result = new StringBuilder();
for (int i = 0; i < n; i++) {
result.append("0");
}
Set visited = new HashSet<>();
visited.add(result.toString());
crackSafeFrom(result, n, k, (int) Math.pow(k, n), visited);
return result.toString();
}
private boolean crackSafeFrom(StringBuilder result, int n, int k, int total, Set visited) {
if (visited.size() == total) {
return true;
}
String curNode = result.substring(result.length() - n + 1);
for (char c = '0'; c < '0' + k; c++) {
if (!visited.contains(curNode + c)) {
result.append(c);
visited.add(curNode + c);
if (crackSafeFrom(result, n, k, total, visited)) {
return true;
}
result.deleteCharAt(result.length() - 1);
visited.remove(curNode + c);
}
}
return false;
} Python:
# Time: O(n * k^n)
# Space: O(n * k^n)
class Solution(object):
def crackSafe(self, n, k):
"""
:type n: int
:type k: int
:rtype: str
"""
def dfs(k, node, lookup, result):
for i in xrange(k):
neighbor = node + str(i)
if neighbor not in lookup:
lookup.add(neighbor)
result.append(str(i))
dfs(k, neighbor[1:], lookup, result)
break
result = [str(k-1)]*(n-1)
lookup = set()
dfs(k, "".join(result), lookup, result)
return "".join(result)
Python:
# Time: O(n * k^n)
# Space: O(n * k^n)
class Solution(object):
def crackSafe(self, n, k):
"""
:type n: int
:type k: int
:rtype: str
"""
result = [str(k-1)]*n
lookup = {"".join(result)}
total = k**n
while len(lookup) < total:
node = result[len(result)-n+1:]
for i in xrange(k):
neighbor = "".join(node) + str(i)
if neighbor not in lookup:
lookup.add(neighbor)
result.append(str(i))
break
return "".join(result)
Python:
# Time: O(k^n)
# Space: O(k^n)
class Solution(object):
def crackSafe(self, n, k):
"""
:type n: int
:type k: int
:rtype: str
"""
M = k**(n-1)
P = [q*k+i for i in xrange(k) for q in xrange(M)] # rotate: i*k^(n-1) + q => q*k + i
result = [str(k-1)]*(n-1)
for i in xrange(k**n):
j = i
# concatenation in lexicographic order of Lyndon words
while P[j] >= 0:
result.append(str(j//M))
P[j], j = -1, P[j]
return "".join(result)
C++:
class Solution {
public:
string crackSafe(int n, int k) {
string res = string(n, '0');
unordered_set visited{{res}};
helper(n, k, pow(k, n), visited, res);
return res;
}
void helper(int n, int k, int total, unordered_set& visited, string& res) {
if (visited.size() == total) return;
string pre = res.substr(res.size() - n + 1, n - 1);
for (int i = k - 1; i >= 0; --i) {
string cur = pre + to_string(i);
if (visited.count(cur)) continue;
visited.insert(cur);
res += to_string(i);
helper(n, k, total, visited, res);
}
}
};
C++:
class Solution {
public:
string crackSafe(int n, int k) {
string res = string(n, '0');
unordered_set visited{{res}};
for (int i = 0; i < n * k; ++i) {
string pre = res.substr(res.size() - n + 1, n - 1);
for (int j = k - 1; j >= 0; --j) {
string cur = pre + to_string(j);
if (!visited.count(cur)) {
visited.insert(cur);
res += to_string(j);
break;
}
}
}
return res;
}
};