Create valid-arrangement-of-pairs.cpp

Author: kamyu104

C++/valid-arrangement-of-pairs.cpp

@@ -0,0 +1,44 @@
+// Time:  O(|V| + |E|)
+// Space: O(|V| + |E|)
+
+// Hierholzer Algorithm
+class Solution {
+public:
+    vector<vector<int>> validArrangement(vector<vector<int>>& pairs) {
+        unordered_map<int, vector<int>> adj;
+        unordered_map<int, int> degree;
+        for (const auto& pair : pairs) {
+            adj[pair[0]].emplace_back(pair[1]);
+            ++degree[pair[0]];
+            --degree[pair[1]];
+        }
+        int x = -1;
+        for (auto& [k, v] : degree) {
+            if (v == 1) {
+                x = k;
+                break;
+            }
+            if (x == -1) {
+                x = k;
+            }
+        }
+        vector<int> route;
+        vector<int> stk = {x};
+        while (!empty(stk)) {
+            while (!empty(adj[stk.back()])) { 
+                auto x = adj[stk.back()].back();
+                adj[stk.back()].pop_back();
+                stk.emplace_back(x);
+                
+            }
+            route.emplace_back(stk.back());
+            stk.pop_back();
+        }
+        reverse(begin(route), end(route));
+        vector<vector<int>> result;
+        for (int i = 0; i + 1 < size(route); ++i) {
+            result.push_back({route[i], route[i + 1]});
+        }
+        return result;
+    }
+};