Create sort_elements_by_frequency.cpp

Author: ishikasinha-d

Diff for: Hashing/sort_elements_by_frequency.cpp

@@ -0,0 +1,81 @@
+// Given an array of integers, sort the array according to frequency of elements. 
+// That is elements that have higher frequency come first. If frequencies of two elements are same, then smaller number comes first.
+// Input:
+// N = 5
+// A[] = {5,5,4,6,4}
+// Output: 4 4 5 5 6
+
+class Solution{
+    public:
+    
+    //custom comparator function to sort map according to frequencies
+    static bool cmp( pair<int, int> &a, pair<int, int> &b)
+    {
+        if(a.second==b.second)
+            return a.first<b.first;
+        return a.second>b.second;
+    }
+   
+    vector<int> sortByFreq(int arr[],int n)
+    {
+        
+        vector<int>ans;
+        map<int,int> m;
+        vector<pair<int, int> > A;
+        
+      //storing the elements as keys and counting its frequency 
+        for(int i=0; i<n; i++)
+        {
+            int x= arr[i];
+            m[x]++;
+        }
+      
+        // Copy key-value pair from Map
+        // to vector of pairs
+        for (auto &it : m) {
+            A.push_back(it);
+        }
+      
+      //sorting the vector of pairs using cmp function
+        sort(A.begin(), A.end(), cmp) ;
+        
+        for(auto x: A)
+        {
+          //printing the element the no. of times it occured in the actual array
+           while(x.second)
+           {
+               ans.push_back(x.first);
+               x.second--;
+           }
+            
+        }   
+        
+        return ans;
+    }
+};
+
+int main() {
+	
+	int t;
+	cin >> t;
+	
+	while(t--){
+	     
+	    int n;
+	    cin >> n;
+	    
+	    int a[n+1];
+	    
+	    for(int i = 0;i<n;i++){
+	        cin >> a[i];
+	    }
+	    Solution obj;
+	    vector<int> v;
+	    v = obj.sortByFreq(a,n);
+	    for(int i:v)
+	        cout<<i<<" ";
+	    cout << endl;
+	}
+	
+	return 0;
+}