Create 2097. Valid Arrangement of Pairs (#647)

Author: Chayandas07

2097. Valid Arrangement of Pairs

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+class Solution {
+public:
+    vector<vector<int>> validArrangement(vector<vector<int>>& pairs) {
+
+        vector<vector<int>> ans;
+        unordered_map<int, int> mp;
+        unordered_map<int, vector<int>> adj;
+
+        for (auto it : pairs) {
+            mp[it[0]]++;
+            mp[it[1]]--;
+            adj[it[0]].push_back(it[1]);
+        }
+
+        //finding start node
+        int starting = pairs[0][0];
+        for (auto it : mp) {
+            if (it.second == 1) {
+                starting = it.first;
+                break;
+            }
+        }
+
+        vector<int> ref;
+        stack<int> st;
+        st.push(starting);
+        //dfs
+        while (st.size()) {
+            auto& nodes = adj[st.top()];
+            if (nodes.empty()) { 
+                ref.push_back(st.top());
+                st.pop();
+            } else {
+                st.push(nodes.back());
+                nodes.pop_back();
+            }
+        }
+        // converting the path
+        for (int i = ref.size() - 1; i > 0; i--) {
+            ans.push_back({ref[i], ref[i - 1]});
+        }
+
+        return ans;
+    }
+};