chore(CPlusPlus): add elements appear thrice (#477)

algorithms/CPlusPlus/Arrays/Elements_appears_thrice.cpp

@@ -0,0 +1,48 @@
+// elements with frequency = (3) in a given array
+//time complexity : O(n) || space complexity : O(1)
+
+#include <bits/stdc++.h>
+#include <iostream>
+
+using namespace std;
+
+int main()
+{ // main function begins here
+    int size;
+    cout << "Enter the size of the array : ";
+    cin >> size;
+
+    int *arr = new int[size];
+    map<int, int> mp;
+
+    for (int i = 0; i < size; i++)
+    {
+        cout << "Enter " << i << " th element of array : ";
+        cin >> arr[i];
+        mp[arr[i]]++; // this simply stores the frequency of each element in this array
+    }
+
+    map<int, int>::iterator it; // this is an iterator which is used to iterate all over the map
+
+    int count = 0;
+
+    for (it = mp.begin(); it != mp.end(); it++)
+    {
+        if ((*it).second == 3) // checks whether element frequency is 3 or not
+        {
+            count++; // just check how many elements are there which have frequency 3
+            if (count == 1)
+            {
+                cout << "The elements whose frequency is 3 are: ";
+            }
+            cout << (*it).first << " "; // print the element which satisfy the above condition
+        }
+    }
+    if (count == 0)
+    {
+        cout << "There is no elements in this array whose frequency is 3."; // special print if there is no such elements
+    }
+
+    cout << endl;
+    return 0;
+}

algorithms/CPlusPlus/README.md

@@ -14,7 +14,8 @@
 10. [Quick Selection](Arrays/quick-select.cpp)
 11. [Remove Duplicates](Arrays/remove-duplicates.cpp)
 12. [Leaders In The Array](Arrays/leaders-in-array.cpp)
-13. [Maximum Difference](Arrays/maximum-difference.cpp)
+13. [Elements appear thrice In The Array](Arrays/Elements_appears_thrice.cpp)
+14. [Maximum Difference](Arrays/maximum-difference.cpp)
 
 ## Dynamic-Programming