Add 3way quick sort for c++ (#85)

* Add 3way quick sort for c++

* Include description for 3way_quick_sort

sorting/README.md

@@ -13,6 +13,7 @@
 9.  [Radix Sort](c-or-cpp/radix-sort.cpp)
 10. [Shell Sort](c-or-cpp/shell-sort.cpp)
 11. [Comb Sort](c-or-cpp/comb-sort.cpp)
+12. [3 Way Quick Sort](c-or-cpp/3way_quick_sort.cpp)
 
 
 

sorting/c-or-cpp/3way_quick_sort.cpp

@@ -0,0 +1,58 @@
+#include <iostream>
+#include <vector>
+#include <cstdlib>
+
+using std::vector;
+using std::swap;
+
+void partition(vector<int> &a, int l, int r, int &i, int &j) {
+  if (r - l <= 1){
+    if (a[r] < a[l])
+      swap (a[r], a[l]);
+
+    i = l;
+    j = r;
+    return;
+  }
+
+  int m = l;
+  int pivot = a[r];
+
+  while (m <= r) {
+    if (a[m] < pivot)
+      swap(a[l++], a[m++]);
+    else if (a[m] == pivot)
+      m++;
+    else {
+      swap (a[r--], a[m]);
+    }
+    i = l-1;
+    j = m;
+  }
+}
+
+void quickSort(vector<int> &a, int l, int r) {
+  if (l >= r) {
+    return;
+  }
+
+  int k = l +(r - l)/2;
+  int i, j;
+  partition(a, l, r, i, j);
+  quickSort(a, l, i);
+  quickSort(a, j, r);
+}
+
+int main() {
+  int n;
+  std::cin >> n;
+  vector<int> a(n);
+  for (size_t i = 0; i < a.size(); ++i) {
+    std::cin >> a[i];
+  }
+  //randomized_quick_sort(a, 0, a.size() - 1);
+  quickSort(a, 0, a.size()-1);
+  for (size_t i = 0; i < a.size(); ++i) {
+    std::cout << a[i] << ' ';
+  }
+}