Create distance-to-a-cycle-in-undirected-graph.cpp

C++/distance-to-a-cycle-in-undirected-graph.cpp

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+// Time:  O(|V| + |E|)
+// Space: O(|V| + |E|)
+
+// graph, dfs, bfs
+class Solution {
+public:
+    vector<int> distanceToCycle(int n, vector<vector<int>>& edges) {
+        vector<vector<int>> adj(n);
+        for (const auto& e : edges) {
+            adj[e[0]].emplace_back(e[1]);
+            adj[e[1]].emplace_back(e[0]);
+        }
+        return bfs(adj, iter_dfs(adj));
+    }
+
+private:
+    vector<int> cycle(const vector<int>& parent, int v, int u) {
+        vector<int> result = {parent[v], v};
+        for (; u != parent[v]; u = parent[u]) {
+            result.emplace_back(u);
+        }
+        return result;
+    }
+
+    vector<int> iter_dfs(const vector<vector<int>>& adj) {
+        vector<int> stk = {0};
+        vector<int> parent(size(adj), -2);
+        parent[0] = -1;
+        while (!empty(stk)) {
+            const int u = stk.back(); stk.pop_back();
+            for (const auto& v : adj[u]) {
+                if (parent[v] != -2) {
+                    if (v == parent[u]) {
+                        continue;
+                    }
+                    return cycle(parent, v, u);
+                }
+                parent[v] = u;
+                stk.emplace_back(v);
+            }
+        }
+        return {};
+    }
+
+    vector<int> bfs(const vector<vector<int>>& adj, vector<int> q) {
+        vector<int> result(size(adj), -1);
+        for (const auto& x : q) {
+            result[x] = 0;
+        }
+        for (int d = 1; !empty(q); ++d) {
+            vector<int> new_q;
+            for (const auto& u : q) {
+                for (const auto& v : adj[u]) {
+                    if (result[v] != -1) {
+                        continue;
+                    }
+                    result[v] = d;
+                    new_q.emplace_back(v);
+                }
+            }
+            q = move(new_q);
+        }
+        return result;
+    }
+};