# BFS solution C++

• Every node has 8 edges. The nodes in dead ends cannot be visited. Find the shortest path from the initial node to the target.

``````class Solution {
public:
int openLock(vector<string>& deadends, string target) {
unordered_set<string> visited;
queue<string> bfs;
string init = "0000";
if (dds.find(init) != dds.end()) return -1;
visited.insert("0000");
bfs.push("0000");
int res = 0;
while (!bfs.empty()) {
int sz = bfs.size();
for (int i = 0; i < sz; i++) {
string t = bfs.front(); bfs.pop();
vector<string> nbrs = move(nbrStrs(t));
for (auto s : nbrs) {
if (s == target) return ++res;
if (visited.find(s) != visited.end()) continue;
if (dds.find(s) == dds.end()) {
bfs.push(s);
visited.insert(s);
}
}
}
++res;
}
return -1;
}

vector<string> nbrStrs(string key) {
vector<string> res;
for (int i = 0 ; i < 4; i++) {
string tmp = key;
tmp[i] = (key[i] - '0' + 1) % 10 + '0';
res.push_back(tmp);
tmp[i] = (key[i] - '0' - 1 + 10) % 10 + '0';
res.push_back(tmp);
}
return res;
}
};
``````

Bidirectional BFS improves the efficiency

``````    int openLock(vector<string>& deadends, string target) {
unordered_set<string> q1, q2, pass, visited;
string init = "0000";
if (dds.find(init) != dds.end() || dds.find(target) != dds.end()) return -1;
visited.insert("0000");
q1.insert("0000"), q2.insert(target);
int res = 0;
while (!q1.empty() && !q2.empty()) {
if (q1.size() > q2.size()) swap(q1, q2);
pass.clear();
for (auto ss : q1) {
vector<string> nbrs = nbrStrs(ss);
for (auto s : nbrs) {
if (q2.find(s) != q2.end()) return res + 1;
if (visited.find(s) != visited.end()) continue;
if (dds.find(s) == dds.end()) {
pass.insert(s);
visited.insert(s);
}
}
}
swap(q1, pass);
res++;
}
return -1;
}
``````

• @enigmaJY did it get accepted? I tried similar approach and it ran into time limit error.

• @nragrawal accepted

• This post is deleted!

• Nice solution. Could you please emphasize what exactly is `move()` doing here? I haven't used/seen it being used ever before. Thanks!

• @lkjhgfdsa It is a move constructor. If you use a copy constuctor there, it will get accepted as well.

• @lkjhgfdsa c++'s std::move

• How come is 16 edges? I thought each digit has three choices: +1, -1, remain the same...
doesn't it?

Shouldn't it be 3^4 - 1 = 80 edges? (81 minus the one situation that all digits remain the same)

• @jackyuan.jack You have 4 circular wheels. In every operation, you choose one wheel to rotate and can rotate towards 2 directions. Should be 4 * 2 edges. Thanks for correction.

• hash set "dds" and "visited" can actually be merged to simplify the code.
Nice solution anyway, especially the bidirectional bfs.

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