You have a lock in front of you with 4 circular wheels. Each wheel has 10 slots: '0', '1', '2', '3', '4', '5', '6', '7', '8', '9'. The wheels can rotate freely and wrap around: for example we can turn '9' to be '0', or '0' to be '9'. Each move consists of turning one wheel one slot.
The lock initially starts at '0000', a string representing the state of the 4 wheels.
You are given a list of deadends dead ends, meaning if the lock displays any of these codes, the wheels of the lock will stop turning and you will be unable to open it.
Given a target representing the value of the wheels that will unlock the lock, return the minimum total number of turns required to open the lock, or -1 if it is impossible.
Example 1:
Input: deadends = ["0201","0101","0102","1212","2002"], target = "0202"
Output: 6
Explanation:
A sequence of valid moves would be "0000" -> "1000" -> "1100" -> "1200" -> "1201" -> "1202" -> "0202".
Note that a sequence like "0000" -> "0001" -> "0002" -> "0102" -> "0202" would be invalid,
because the wheels of the lock become stuck after the display becomes the dead end "0102".
Example 2:
Input: deadends = ["8888"], target = "0009"
Output: 1
Explanation:
We can turn the last wheel in reverse to move from "0000" -> "0009".
Example 3:
Input: deadends = ["8887","8889","8878","8898","8788","8988","7888","9888"], target = "8888"
Output: -1
Explanation:
We can't reach the target without getting stuck.
Example 4:
Input: deadends = ["0000"], target = "8888"
Output: -1
Note:
- The length of
deadends will be in the range[1, 500]. -
target will not be in the listdeadends. - Every string in
deadends and the stringtarget will be a string of 4 digits from the 10,000 possibilities'0000' to'9999'.
题解：
bfs，使用visit存储已访问的状态，再用比ans大的路径不进行访问实现剪枝。
最快的解法是stoi然后同样bfs。
class Solution {
public:
int openLock(vector<string>& deadends, string target) {
set<string> dead(deadends.begin(), deadends.end());
string init = "0000";
set<string> visit;
queue<pair<int, string>> q;
int ans = INT_MAX;
if (dead.count(init) == true) {
return -1;
}
q.push({0, init});
visit.insert(init);
while (q.empty() == false) {
auto tmp = q.front();
q.pop();
if (tmp.second == target) {
ans = min(ans, tmp.first);
continue;
}
if (tmp.first > ans) {
continue;
}
for (int i = 0; i < 4; i++) {
string up = tmp.second, down = tmp.second;
if (up[i] == '9') {
up[i] = '0';
}
else {
up[i]++;
}
if (down[i] == '0') {
down[i] ='9';
}
else {
down[i]--;
}
if (visit.count(up) == false && dead.count(up) == false) {
q.push({tmp.first + 1, up});
visit.insert(up);
}
if (visit.count(down) == false && dead.count(down) == false) {
q.push({tmp.first + 1, down});
visit.insert(down);
}
}
}
if (ans == INT_MAX) {
return -1;
}
return ans;
}
};
