LeetCode Weekly Contest 64 - 752. Open the Lock【BFS + string】

752. Open the Lock

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 list deadends.

Every string in deadends and the string target will be a string of 4 digits from the 10,000 possibilities ‘0000’ to ‘9999’.

题意： 给你一个四位的密码锁，告诉你开锁的密码，同时也告诉你一些密码不可以经过，让你求最少的步数来打开锁，不能的话返回-1.

分析： 一开始还以为是贪心，后来看了正解后才反映过来（做题太少啦），看到BFS后，瞬间有了思路，最近没做过BFS，只不过加了一些不可以走的点，和二维表中的BFS思路一样，爆搜即可

参考函数

class Solution {
public:
int openLock(vector<string>& deadends, string target) {
set<string> ds;
map<string,int> dis;
for(string s: deadends) ds.insert(s);
queue<string> q;
q.push("0000");
dis["0000"] = 0;
while (q.size()) {
string s1 = q.front();q.pop();
if(ds.count(s1)) continue;
for(int i = 0;i < 4;i++) {
for(int j = -1;j < 2;j += 2) {
string s2 = s1;
s2[i] += j;
if(s2[i] > '9') s2[i] -= 10;
if(s2[i] < '0') s2[i] += 10;
if(ds.count(s2)) continue;
if(dis[s2]) continue;
q.push(s2);
dis[s2] = dis[s1] + 1;
}
}
}
if(dis.count(target)) return dis[target];
return -1;
}
};

如有错误或遗漏，请私聊下UP，thx