题目原文
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
deadendswill be in the range[1, 500]. targetwill not be in the listdeadends.- Every string in
deadendsand the stringtargetwill be a string of 4 digits from the 10,000 possibilities'0000'to'9999'.
题意分析
转自 ： here
从初始状态“0000”开始转动，每次只能将其中一位转动一格，可正转可反转，同时deadends中有多个状态，如果转到该状态则不能继续，也即必不可转到目标结果。求问转到目标结果的最小次数。解法分析
本题需要找到一个最小次数，通常需要运用BFS，在BFS中加入剪枝，并且对于到过的状态建立unordered_set，通过上述方法减少遍历次数。C++代码如下：class Solution {
public:
/* static bool isEnd(string a,vector<string>& deadends){
for(auto k:deadends){
if(a==k)
return 1;
}
return 0;
}*/
int openLock(vector<string>& deadends, string target) {
queue<string> myQue;
unordered_set<string> dde(deadends.begin(),deadends.end());
if(dde.count("0000"))
return -1;
myQue.push("0000");
int queSize=1;
int layer=0;
int i,j;
string temp,temp1;
unordered_set<string> mySet;
mySet.insert("0000");
while(!myQue.empty()){
layer++;
queSize=myQue.size();
for(i=0;i<queSize;i++){
temp=myQue.front();
for(j=0;j<4;j++){
temp1=temp;//add
if(temp1[j]=='9')
temp1[j]='0';
else
temp1[j]+=1;
if(temp1==target)
return layer;
if(!dde.count(temp1)&&!mySet.count(temp1)){
myQue.push(temp1);
mySet.insert(temp1);
}
temp1=temp;//sub
if(temp1[j]=='0')
temp1[j]='9';
else
temp1[j]-=1;
if(temp1==target)
return layer;
if(!dde.count(temp1)&&!mySet.count(temp1)){
myQue.push(temp1);
mySet.insert(temp1);
}
}
myQue.pop();
}
}
return -1;
}
};
