Added quicksort C implementation (#16)

* feat: added quicksort c implementation

* docs: added entry to README

* chore: optimized code

* refactor: added comments

* refactor: refactored code

README.md

@@ -9,6 +9,7 @@ Community (college) maintained list of algorithm implementations and codes.
 | Algorithm | C | CPP | Java | Python |
 |:--------------|:----------------:|:----------------:|:----------------:|:-----------------:|
 | [Linear Search](https://en.wikipedia.org/wiki/Linear_search) | | | | [\[X\]](linear_search/linear_search.py) |
+| [Quicksort](https://en.wikipedia.org/wiki/Quicksort) | [\[X\]](quicksort/quicksort.c) | | | |
 
 
 ## Resources

quicksort/quicksort.c

@@ -0,0 +1,59 @@
+#include "stdio.h"
+
+/*
+ * @ar_size array size
+ * @ar array pointer
+ */
+void quicksort(int ar_size, int * ar) {
+	int piv;
+	piv = ar[ar_size-1]; // chose last element as pivot
+	int i, temp, c = 0;
+	int prevmax, prevind = -1; // stores last number >= pivot
+
+	for (i=0; i<ar_size-1; i++){
+		if (ar[i] < piv){
+			if ((prevind > -1) && (ar[i] < prevmax)){
+				// swap current and previous large element
+				// current element is less than prevmax so move it to the left side
+				// and prevmax to the right side
+				temp = ar[i];
+				ar[i] = prevmax;
+				ar[prevind] = temp;
+				// prevind now has a less than pivot element, no need for it now
+				// therefore increment it for the next number
+				prevind = prevind + 1;
+				prevmax = ar[prevind];
+			}
+			c++;  // pivot's final location (depends on count of items smaller than pivot)
+		}
+		else if (prevind < 0) {
+			// init first larger than pivot element
+			// it may be swapped out later (above block)
+			prevmax = ar[i];
+			prevind = i;
+		}
+	}
+
+	if (prevind > -1){ // if some swaps were done
+		ar[ar_size-1] = ar[c];
+		ar[c] = piv; // swap pivot to its correct position
+	}
+
+	if (ar_size > 2){
+		if (c != 0)
+			quicksort(c, &ar[0]); // recursively quicksort left partition
+		if (ar_size-c-1 != 0)
+			quicksort(ar_size-c-1, &ar[c+1]); // right partition
+	}
+}
+
+int main() {
+	int ar_size = 4, i;
+	int a[4] = {2, 3, 0, 4};
+	quicksort(ar_size, a);
+
+	for (i=0; i<ar_size; i++){
+		printf("%d\n", a[i]);
+	}
+	return 0;
+}