Create sorting_by_frequency.cpp

sorting_by_frequency.cpp

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+  /* 
+  Sorting array by frequency in decreasing order
+  Program to Print the elements of an array in the decreasing frequency if 2 numbers have same frequency then print the one which came first. 
+
+  Input:  arr[] = {2, 5, 2, 8, 5, 6, 8, 8}
+
+    After sorting we get
+    Element 2 2 5 5 6 8 8 8
+    Index   0 2 1 4 5 3 6 7
+
+    Now construct the 2D array as
+    Index, Count
+    0,      2
+    1,      2
+    5,      1
+    3,      3
+
+    Sort by count (consider indexes in case of tie)
+    3, 3
+    0, 2
+    1, 2
+    5, 1
+
+    Print the elements using indexes in the above 2D array.
+    Output: arr[] = {8, 8, 8, 2, 2, 5, 5, 6}
+
+  Using a hashing mechanism, we can store the elements (also first index) and their counts in a hash. Finally, sort the hash elements according to their counts.
+  Below is the implementation of above approach – 
+
+  */
+
+#include <iostream>
+#include <vector>
+#include <unordered_map>
+#include <algorithm>
+
+// Compare function
+bool fcompare(std::pair<int, std::pair<int, int> > p,
+			std::pair<int, std::pair<int, int> > p1)
+{
+	if (p.second.second != p1.second.second)
+		return (p.second.second > p1.second.second);
+	else
+		return (p.second.first < p1.second.first);
+}
+void sortByFrequency(int arr[], int n)
+{
+	std::unordered_map<int, std::pair<int, int> > hash; // hash map
+	for (int i = 0; i < n; i++) {
+		if (hash.find(arr[i]) != hash.end())
+			hash[arr[i]].second++;
+		else
+			hash[arr[i]] = std::make_pair(i, 1);
+	} // store the count of all the elements in the hashmap
+
+	// Iterator to Traverse the Hashmap
+	auto it = hash.begin();
+
+	// Vector to store the Final Sortted order
+	std::vector<std::pair<int, std::pair<int, int> > > b;
+	for (it; it != hash.end(); ++it)
+		b.push_back(std::make_pair(it->first, it->second));
+
+	sort(b.begin(), b.end(), fcompare);
+
+	// output
+	for (int i = 0; i < b.size(); i++) {
+		int count = b[i].second.second;
+		while (count--)
+			std::cout << b[i].first << " ";
+	}
+}
+
+int main()
+{
+    int n;
+
+    std::cout << "Enter the amount of numbers to sort: ";
+    std::cin >> n;
+    int arr[n];
+    std::cout << "Enter " << n << " numbers: ";
+    int num;
+
+
+    for (int i = 0; i < n; i++) {
+        std::cin >> num;
+        arr[i] = num;
+    }
+
+
+	sortByFrequency(arr, n);
+
+	return 0;
+}