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Quick Sort Implementation #2 that put logic of partition into a separate function which return partition index
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| public class QuickSort2 { | |
| /** | |
| * @param A: an integer array | |
| * @return: nothing | |
| */ | |
| public void sortIntegers2(int[] A) { | |
| if (A == null || A.length <= 1) return; | |
| quicksort(A, 0, A.length - 1); | |
| } | |
| private void quicksort(int[] A, int start, int end) { | |
| if (start >= end) return; | |
| int idx = parition(A, start, end); | |
| quicksort(A, start, idx - 1); | |
| quicksort(A, idx + 1, end); | |
| } | |
| // first way: initially putting pivot at the end | |
| private void parition(int[] A, int start, int end) { | |
| // here I am using the middle index as the pivot | |
| int mid = start + (end - start) / 2; | |
| int pivot = A[mid]; | |
| // move the pivot to the end of subarray | |
| swap(A, mid, end); | |
| int storeIdx = l; | |
| // iterate through subarray [start, end - 1], moving all elements that | |
| // are less than the pivot to the left of i (which is marked by storeIdx) | |
| for (int i = start; i < end; i++) { | |
| if (A[i] < pivot) { | |
| swap(A, i, storeIdx++); | |
| } | |
| } | |
| // swap pivot with A[storeIdx] | |
| swap(A, storeIdx, end); | |
| // Invariant: | |
| // A[start, storeIdx - 1]: elements are less than pivot | |
| // A[storeIdx]: pivot | |
| // A[storeIdx + 1, end]: elements that are greater than or equal to pivot | |
| return storeIdx; | |
| } | |
| // second way initally putting pivot at the beginning | |
| private int partition(int[] A, int start, int end) { | |
| int mid = start + (end - start) / 2; | |
| int pivot = A[mid]; | |
| // swap pivot to the front | |
| swap(A, mid, start); | |
| int i = start + 1, j = start + 1; | |
| while (i <= end) { | |
| if (A[i] < pivot) { | |
| swap(A, i, j++); | |
| } | |
| i++; | |
| } | |
| swap(A, start, j - 1); | |
| return j - 1; | |
| } | |
| private void swap(int[] A, int i, int j) { | |
| int temp = A[i]; | |
| A[i] = A[j]; | |
| A[j] = temp; | |
| } | |
| } |
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