Update find-if-path-exists-in-graph.cpp

Author: kamyu104

C++/find-if-path-exists-in-graph.cpp

@@ -67,11 +67,11 @@ class Solution2 {
     int bfs(const unordered_map<int, vector<int>>& adj,
                const int start,
                const int target) {
-        unordered_set<int> q = {start};
+        vector<int> q = {start};
         unordered_set<int> lookup;
         int steps = 0;
         while (!empty(q)) {
-            unordered_set<int> new_q;
+            vector<int> new_q;
             for (const auto& pos : q) {
                 if (pos == target) {
                     return steps;
@@ -84,7 +84,7 @@ class Solution2 {
                         continue;
                     }
                     lookup.emplace(nei);
-                    new_q.emplace(nei);
+                    new_q.emplace_back(nei);
                 }
             }
             q = move(new_q);
@@ -93,3 +93,43 @@ class Solution2 {
         return -1;
     }
 };
+
+// Time:  O(|V| + |E|)
+// Space: O(|V| + |E|)
+// dfs solution
+class Solution3 {
+public:
+    bool validPath(int n, vector<vector<int>>& edges, int start, int end) {
+        unordered_map<int, vector<int>> adj;
+        for (const auto& edge : edges) {
+            adj[edge[0]].emplace_back(edge[1]);
+            adj[edge[1]].emplace_back(edge[0]);
+        }
+        return dfs(adj, start, end);
+    }
+
+private:
+    int dfs(const unordered_map<int, vector<int>>& adj,
+               const int start,
+               const int target) {
+        vector<int> stk = {start};
+        unordered_set<int> lookup;
+        while (!empty(stk)) {
+            auto pos = stk.back(); stk.pop_back();
+            if (pos == target) {
+                return true;
+            }
+            if (!adj.count(pos)) {
+                continue;
+            }
+            for (const auto& nei : adj.at(pos)) {
+                if (lookup.count(nei)) {
+                    continue;
+                }
+                lookup.emplace(nei);
+                stk.emplace_back(nei);
+            }
+        }
+        return false;
+    }
+};