Create QuickSort.c

SortingAlgorithms/quickSort/QuickSort.c

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+#include <stdio.h>
+#include <stdlib.h>
+#define N 20
+
+void printSeq(int a[], int n) {
+  printf("[");
+  for (int i=0; i<n; i++) printf("%d ", a[i]);
+  if (n==0) printf("]\n"); else printf("\b]\n");
+}
+
+void quicksort(int arr[], int n) {
+/* Sorts arr in ascending order.
+ * n is the dimension of arr.
+ * Uses a divide and conquer approach.
+ * 1. Split arr into two parts left and right with respect
+ * to a pivot where left contains elements < pivot and
+ * right has elements >= pivot.  This is the divide step.
+ * 2. quicksort left, quicksort right. The recursive step.
+ * 3. stick left, pivot, right for final sorted array - conquer step.
+ */
+  if(n<2) return;//base step. Array sorted if empty or has 1 element.
+  int larr[n], rarr[n], pivot=arr[0];//pivot chosen as first element in arr.
+  int i, li=0, ri=0;
+//Divide step
+  for(i=1;i<n;i++)
+    if(arr[i]<pivot) larr[li++]=arr[i]; else rarr[ri++]=arr[i];
+  printf("larr="); printSeq(larr,li); printf("pivot=%d\n", pivot);
+  printf("rarr=");printSeq(rarr,ri);
+//Recursive step
+  quicksort(larr,li); quicksort(rarr,ri);
+//Conquer step. Copy in order arrays larr, pivot and rarr into arr.
+  for(i=0;i<li;i++) arr[i]=larr[i];//copy larr
+  arr[i]=pivot;//copy pivot
+  for(i++;i<n;i++) arr[i]=rarr[i-li-1];//copy rarr
+  return;
+}
+int main() {
+  int a[N], n, i;
+  printf("Sequence length (>0, <20) = ");
+  scanf("%d", &n);
+  if (n<1) {printf("Illegal length\n"); exit(0);}
+  printf("Give seq. elements sep. by white space = ");
+  for (i=0; i<n; i++) scanf("%d", &a[i]);
+  printf("Original sequence = ");
+  printSeq(a, n);
+  quicksort(a, n);
+  printf("Sorted sequence = ");
+  printSeq(a, n);
+  return 0;
+}