Add k most frequent elements using quick select

Author: rp-code9

README.md

@@ -96,7 +96,7 @@ Short sample programs to demonstrate the inbuilt object oriented concepts in Jav
 
 | Maths | Link | Java  | C++|
 |:--|:--|:--|:--|
-| Top K frequent elements | [Link](https://leetcode.com/explore/interview/card/top-interview-questions-medium/110/sorting-and-searching/799/) | ? | [Using hashmap and sorting](sites/leetcode/FrequentElements.cpp) <br/> [Using hashmap and priority queue](sites/leetcode/FrequentElements.cpp) |
+| Top K frequent elements | [Link](https://leetcode.com/explore/interview/card/top-interview-questions-medium/110/sorting-and-searching/799/) | ? | [Using hashmap and sorting](sites/leetcode/FrequentElements.cpp) <br/> [Using hashmap and priority queue](sites/leetcode/FrequentElements.cpp) <br/> [Using Quick Select](sites/leetcode/FrequentElementsQuickSelect.cpp) |
 |  | [Link]() | ? | []() |
 
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sites/leetcode/FrequentElementsQuickSelect.cpp

@@ -0,0 +1,93 @@
+// Top K frequent elements
+// https://leetcode.com/explore/interview/card/top-interview-questions-medium/110/sorting-and-searching/799/
+
+#include <iostream>
+#include <vector>
+#include <algorithm>
+#include <unordered_map>
+#include <cstdlib>
+
+void print(const std::vector<int>& numbers)
+{
+    if (numbers.empty()) {
+        return;
+    }
+    for (auto num : numbers) {
+        std::cout << num << " ";
+    }
+    std::cout << "\n";
+}
+
+int partition(const int left, const int right, const int kSmallest,
+    std::vector<int>& uniqueKeys, std::unordered_map<int, int>& freqMap)
+{
+    int pivotFrequency = freqMap[uniqueKeys[kSmallest]];
+    std::swap(uniqueKeys[kSmallest], uniqueKeys[right]);
+
+    int storeIndex = left;
+    for (int i = left; i <= right; ++i) {
+        if (freqMap[uniqueKeys[i]] < pivotFrequency) {
+            std::swap(uniqueKeys[i], uniqueKeys[storeIndex]);
+            storeIndex += 1;
+        }
+    }
+    std::swap(uniqueKeys[right], uniqueKeys[storeIndex]);
+    return storeIndex;
+}
+
+void quickselect(const int left, const int right, const int kSmallest,
+    std::vector<int>& uniqueKeys, std::unordered_map<int, int>& freqMap)
+{
+    if (left == right) {
+        return;
+    }
+
+    int pivotIndex = left + std::rand() % (right - left + 1);
+    pivotIndex = partition(left, right, pivotIndex, uniqueKeys, freqMap);
+    if (pivotIndex == kSmallest) {
+        return;
+    } else if (kSmallest < pivotIndex) {
+        quickselect(left, pivotIndex - 1, kSmallest, uniqueKeys, freqMap);
+    } else {
+        quickselect(pivotIndex + 1, right, kSmallest, uniqueKeys, freqMap);
+    }
+}
+
+std::vector<int> topKFrequent(std::vector<int>& numbers, const int k)
+{
+    std::unordered_map<int, int> freqMap;
+
+    for (auto num : numbers) {
+        freqMap[num]++;
+    }
+
+    size_t n = freqMap.size();
+    std::vector<int> uniqueKeys {};
+    for (auto p : freqMap) {
+        uniqueKeys.push_back(p.first);
+    }
+
+    quickselect(0, n - 1, n - k, uniqueKeys, freqMap);
+    std::vector<int> kFrequent(k);
+    std::copy(uniqueKeys.begin() + n - k, uniqueKeys.end(), kFrequent.begin());
+
+    return kFrequent;
+}
+
+int main()
+{
+    std::vector<int> numbers {3, 1, 4, 4, 5, 2, 6, 1};
+    std::cout << "Numbers : ";
+    print(numbers);
+
+    std::cout << "---------------------------------------\n";
+    int k = 2;
+    std::cout << "Top "<< k << " frequent numbers : \n";
+
+    std::vector<int> frequent = topKFrequent(numbers, k);
+    print(frequent);
+
+    std::cout << "=======================================\n";
+
+    return 0;
+}